Spell check with typos. Add spell-check CI job

[vvv]

Other changes

Fix the unit tests

Update length_expr_parentheses.stderr to let tests/compile-fail/length_expr_parentheses.rs pass.

Console output (before this patch is applied):

test tests/compile-fail/length_expr_parentheses.rs [should fail to compile] ... mismatch

EXPECTED:
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
error: this file contains an unclosed delimiter
  --> tests/compile-fail/length_expr_parentheses.rs:15:1
   |
15 | #[packet]
   | ^^^^^^^^^ unclosed delimiter
   |
   = note: this error originates in the derive macro `::pnet_macros::Packet` (in Nightly builds, run with -Z macro-backtrace for more info)
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈

ACTUAL OUTPUT:
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
error: this file contains an unclosed delimiter
  --> tests/compile-fail/length_expr_parentheses.rs:15:1
   |
15 | #[packet]
   | ^^^^^^^^^ unclosed delimiter
   |
   = note: this error originates in the derive macro `::pnet_macros::Packet` (in Nightly builds, run with -Z macro-backtrace for more info)

error: proc-macro derive panicked
  --> tests/compile-fail/length_expr_parentheses.rs:15:1
   |
15 | #[packet]
   | ^^^^^^^^^
   |
   = help: message: compiler/fallback mismatch
   = note: this error originates in the attribute macro `packet` (in Nightly builds, run with -Z macro-backtrace for more info)
┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈
note: If the actual output is the correct output you can bless it by rerunning
      your test with the environment variable TRYBUILD=overwrite

Fix "irrefutable while let pattern" warning

warning: irrefutable `while let` pattern
  --> examples/arp_packet.rs:61:11
   |
61 |     while let buf = receiver.next().unwrap() {
   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this pattern will always match, so the loop will never exit
   = help: consider instead using a `loop { ... }` with a `let` inside it
   = note: `#[warn(irrefutable_let_patterns)]` on by default

[mrmonday]

Looks good, thanks!