These functions get called for a pointer because that is what we use to
implement the LogSink interface. The variant of these functions which take a
value instead of a pointer was never used, so providing directly what is needed
instead of relying on the compiler to generate that is more efficient (less
code, avoids one extra callstack entry at runtime).

The With* functions must continue to take a value because they need to modify
that copy.

This makes it possible to use test.Output inside packages
that have other tests.

When klogr called V(), it didn't pass the callstack offset, which broke
evaluation of the -vmodule patterns. A new VWithOffset klog API is needed to
fix this.

A unit test for this will follow.

Besides sharing code, it's also better in two regards:
- restores state after test
- avoids triple printing of errors

When Logger.Info called Enabled, the wrong number of stack frames were skipped.

A unit test for this will follow.

Previously it was necessary to enter the "examples" module to run output tests
for code in the main module. Now "go test ./..." at the root or in individual
directories also runs these tests.

So far, the output tests were only used for correctness checking. But they
cover a variety of scenarios for which there were no benchmarks, therefore it
makes sense to also use them for that.

Text logger and the two klogr implementations both got this wrong.

This simplifies the code. Instead of different instances, the package now
maintains a global pool. This makes the text logger struct a bit smaller and
thus cheaper to copy in the With* functions.

Performance is about the same as before:

name              old time/op    new time/op    delta
Header-36           1.68µs ± 7%    1.62µs ± 6%   ~     (p=0.246 n=5+5)
HeaderWithDir-36    1.63µs ± 6%    1.59µs ± 6%   ~     (p=0.690 n=5+5)

name              old alloc/op   new alloc/op   delta
Header-36             216B ± 0%      216B ± 0%   ~     (all equal)
HeaderWithDir-36      216B ± 0%      216B ± 0%   ~     (all equal)

name              old allocs/op  new allocs/op  delta
Header-36             2.00 ± 0%      2.00 ± 0%   ~     (all equal)
HeaderWithDir-36      2.00 ± 0%      2.00 ± 0%   ~     (all equal)

The text logger didn't actually return the buffer. Now it does.

This avoids one memory allocation (for the intermediate slice), copying and
loops.

Using a strings.Builder reduces the number of allocations:

name                               old time/op    new time/op    delta
KlogOutput/KObjSlice_okay-36         15.2µs ± 5%    14.8µs ± 4%    ~     (p=0.151 n=5+5)
KlogOutput/KObjSlice_nil_entry-36    14.4µs ± 5%    13.6µs ± 3%  -5.25%  (p=0.032 n=5+5)
KlogOutput/KObj-36                   13.5µs ± 8%    13.5µs ± 6%    ~     (p=0.841 n=5+5)
KlogOutput/KObjSlice_ints-36         15.3µs ± 5%    15.2µs ± 4%    ~     (p=0.841 n=5+5)
KlogOutput/KObjSlice_nil_arg-36      12.8µs ± 2%    12.8µs ± 6%    ~     (p=0.841 n=5+5)
KlogOutput/KObjSlice_int_arg-36      14.1µs ± 4%    13.8µs ± 3%    ~     (p=0.310 n=5+5)
KlogOutput/KObjs-36                  14.1µs ± 8%    13.6µs ± 8%    ~     (p=0.690 n=5+5)

name                               old alloc/op   new alloc/op   delta
KlogOutput/KObjSlice_okay-36         2.89kB ± 0%    2.82kB ± 0%  -2.23%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_entry-36    2.65kB ± 0%    2.62kB ± 0%    ~     (p=0.079 n=4+5)
KlogOutput/KObj-36                   2.50kB ± 0%    2.47kB ± 0%  -1.30%  (p=0.000 n=4+5)
KlogOutput/KObjSlice_ints-36         2.90kB ± 0%    2.90kB ± 0%    ~     (p=1.000 n=5+5)
KlogOutput/KObjSlice_nil_arg-36      2.41kB ± 0%    2.41kB ± 0%    ~     (all equal)
KlogOutput/KObjSlice_int_arg-36      2.67kB ± 0%    2.67kB ± 0%    ~     (all equal)
KlogOutput/KObjs-36                  2.72kB ± 0%    2.65kB ± 0%  -2.38%  (p=0.008 n=5+5)

name                               old allocs/op  new allocs/op  delta
KlogOutput/KObjSlice_okay-36           46.0 ± 0%      42.0 ± 0%  -8.70%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_entry-36      40.0 ± 0%      38.0 ± 0%  -5.00%  (p=0.008 n=5+5)
KlogOutput/KObj-36                     36.0 ± 0%      34.0 ± 0%  -5.56%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_ints-36           39.0 ± 0%      39.0 ± 0%    ~     (all equal)
KlogOutput/KObjSlice_nil_arg-36        35.0 ± 0%      35.0 ± 0%    ~     (all equal)
KlogOutput/KObjSlice_int_arg-36        37.0 ± 0%      37.0 ± 0%    ~     (all equal)
KlogOutput/KObjs-36                    42.0 ± 0%      38.0 ± 0%  -9.52%  (p=0.008 n=5+5)

The klog text format avoids some string allocation by calling WriteText instead
of String when encountering such values.

A positive side effect is that KObjSlice now gets logged like KObjs, i.e.

   pods=[namespace1/name1 namespace2/name2]

Previously, it was written as a quoted string.

To achieve the best performance, WriteText is passed a bytes.Buffer pointer
instead of a io.Writer. This avoids an interface allocation for the parameter
and provides access to the underlying methods.

Benchmarks involving these types benefit while other are the same as before:

name                               old time/op    new time/op    delta
KlogOutput/KObj-36                   12.7µs ±10%    13.1µs ± 1%     ~     (p=0.151 n=5+5)
KlogOutput/KObjs-36                  13.4µs ± 7%    14.0µs ± 5%     ~     (p=0.310 n=5+5)
KlogOutput/KObjSlice_okay-36         14.8µs ± 4%    13.0µs ± 3%  -12.33%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_int_arg-36      14.0µs ± 6%    12.3µs ±10%  -12.60%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_ints-36         15.5µs ± 4%    12.8µs ± 6%  -17.85%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_entry-36    14.2µs ±13%    12.6µs ± 3%  -11.39%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_arg-36      12.6µs ± 6%    12.9µs ± 3%     ~     (p=0.690 n=5+5)

name                               old alloc/op   new alloc/op   delta
KlogOutput/KObj-36                   2.47kB ± 0%    2.41kB ± 0%   -2.29%  (p=0.008 n=5+5)
KlogOutput/KObjs-36                  2.65kB ± 0%    2.65kB ± 0%     ~     (all equal)
KlogOutput/KObjSlice_okay-36         2.82kB ± 0%    2.45kB ± 0%  -13.37%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_int_arg-36      2.67kB ± 0%    2.45kB ± 0%   -8.42%  (p=0.029 n=4+4)
KlogOutput/KObjSlice_ints-36         2.90kB ± 0%    2.49kB ± 0%  -14.37%  (p=0.000 n=4+5)
KlogOutput/KObjSlice_nil_entry-36    2.62kB ± 0%    2.43kB ± 0%   -7.34%  (p=0.000 n=4+5)
KlogOutput/KObjSlice_nil_arg-36      2.41kB ± 0%    2.40kB ± 0%   -0.66%  (p=0.016 n=5+4)

name                               old allocs/op  new allocs/op  delta
KlogOutput/KObj-36                     34.0 ± 0%      32.0 ± 0%   -5.88%  (p=0.008 n=5+5)
KlogOutput/KObjs-36                    38.0 ± 0%      38.0 ± 0%     ~     (all equal)
KlogOutput/KObjSlice_okay-36           42.0 ± 0%      32.0 ± 0%  -23.81%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_int_arg-36        37.0 ± 0%      32.0 ± 0%  -13.51%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_ints-36           39.0 ± 0%      33.0 ± 0%  -15.38%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_entry-36      38.0 ± 0%      32.0 ± 0%  -15.79%  (p=0.008 n=5+5)
KlogOutput/KObjSlice_nil_arg-36        35.0 ± 0%      32.0 ± 0%   -8.57%  (p=0.008 n=5+5)

It looks like golangci-lint comes with a fixed version of Go, one which is too
old for the code:

level=error msg="Running error: gofmt: analysis skipped: errors in package: [/go/src/k8s.io/klog/test/output.go:846:22: Discard not declared by package io /go/src/k8s.io/klog/test/output.go:864:40: Discard not declared by package io /go/src/k8s.io/klog/test/output.go:876:35: Discard not declared by package io]"

io.Discard was introduced in Go 1.16, using it should be fine and is reflected
in our Go version test matrix (>= 1.17).