transformers/grep: avoid allocations with (*regexp.Regexp).MatchString

[Juneezee]

We should use (*regexp.Regexp).MatchString instead of (*regexp.Regexp).Match([]byte(...)) when matching string to avoid unnecessary []byte conversions and reduce allocations. We have been using MatchString in other places, e.g. the cut transformer:

miller/pkg/transformers/cut.go

Line 273 in 6aab161

if regex.MatchString(pe.Key) {

Example benchmark:

var grepRegex = regexp.MustCompile("foo.*")

func BenchmarkMatch(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := grepRegex.Match([]byte("foo bar baz")); !match {
			b.Fail()
		}
	}
}

func BenchmarkMatchString(b *testing.B) {
	for i := 0; i < b.N; i++ {
		if match := grepRegex.MatchString("foo bar baz"); !match {
			b.Fail()
		}
	}
}

goos: linux
goarch: amd64
pkg: github.com/johnkerl/miller/pkg/transformers
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 5700908	       210.3 ns/op	      16 B/op	       1 allocs/op
BenchmarkMatchString-16    	 8006731	       156.4 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/johnkerl/miller/pkg/transformers	2.857s

[johnkerl]

Thanks @Juneezee !!!! :D