perf: add initial benchmark

[mccutchen]

Results from my laptop:

$ make bench
go test -bench=. -benchmem
goos: darwin
goarch: arm64
pkg: github.com/mccutchen/websocket
cpu: Apple M2
BenchmarkReadFrame/64b-8                11431972               101.1 ns/op           120 B/op          4 allocs/op
BenchmarkReadFrame/256b-8                5405172               222.2 ns/op           312 B/op          5 allocs/op
BenchmarkReadFrame/1024b-8               1801424               667.1 ns/op          1080 B/op          5 allocs/op
BenchmarkReadFrame/65536b-8                29114             41176 ns/op           65601 B/op          5 allocs/op
BenchmarkReadFrame/1048576b-8               1804            664341 ns/op         1048655 B/op          5 allocs/op
PASS
ok      github.com/mccutchen/websocket  7.914s

More thorough benchmarks will likely require additional refactoring.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.48%. Comparing base (c335860) to head (45f9e80).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #13   +/-   ##
=======================================
  Coverage   79.48%   79.48%           
=======================================
  Files           2        2           
  Lines         424      424           
=======================================
  Hits          337      337           
  Misses         63       63           
  Partials       24       24           

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