Minimize allocations in strutil

[sethvargo]

I noticed some pretty quick-and-easy wins for performance in the strutil package.

1. Use struct{} instead of bool for map membership. The empty struct does not allocate and consumes no memory, so it's far more efficient than a boolean:

   package main
   
   import (
   	"fmt"
   	"unsafe"
   )
   
   func main() {
   	x := [100]struct{}{}
   	y := [100]bool{}
   
   	fmt.Printf("%v\n", unsafe.Sizeof(x)) // 0
   	fmt.Printf("%v\n", unsafe.Sizeof(y)) // 100
   }

2. Pre-allocate slices and maps when we know the maximum possible size.

3. Remove an unnecessary test dependency and rename the tests to match Go's expected test format name.

Benchmarks

Before:

BenchmarkRemoveDuplicates-16          	       2	 756790682 ns/op	106821536 B/op	   38192 allocs/op
BenchmarkRemoveDuplicatesStable-16    	       6	 183990038 ns/op	58343824 B/op	       4 allocs/op
BenchmarkEquivalentSlices-16          	       1	1666107197 ns/op	358676792 B/op	   76680 allocs/op

After:

BenchmarkRemoveDuplicates-16          	       3	 477817660 ns/op	56117237 B/op	       7 allocs/op
BenchmarkRemoveDuplicatesStable-16    	       6	 200917214 ns/op	56115224 B/op	       3 allocs/op
BenchmarkEquivalentSlices-16          	       1	1009316412 ns/op	112230496 B/op	       8 allocs/op

[sethvargo]

Pokey @jefferai @calvn

[jefferai]

I'm on vacation :-)

[jefferai]

Thanks!

[sethvargo]

Thank you 😄