var-naming: 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. A one-line change for free performance improvement.

Example benchmark:

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

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

goos: linux
goarch: amd64
pkg: github.com/mgechev/revive/rule
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	 5248138	       285.8 ns/op	       8 B/op	       1 allocs/op
BenchmarkMatchString-16    	 6899334	       170.9 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/mgechev/revive/rule	4.102s

[chavacava]

Thank you @Juneezee, nice catch!