goat: avoid unnecessary byte/string conversion

[Juneezee]

We can use (*regexp.Regexp).MatchString instead of (*regexp.Regexp).Match([]byte(...)) to avoid unnecessary []byte conversions and reduce allocations.

Example benchmark:

package main_test

import (
	"regexp"
	"testing"
)

var testRegex = regexp.MustCompile(`\w+`)

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

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

Result:

goos: linux
goarch: amd64
pkg: github.com/zhenghaoz/gorse/cmd/goat
cpu: AMD Ryzen 7 PRO 4750U with Radeon Graphics
BenchmarkMatch-16          	11826721	       138.9 ns/op	       3 B/op	       1 allocs/op
BenchmarkMatchString-16    	12954583	        88.85 ns/op	       0 B/op	       0 allocs/op
PASS
ok  	github.com/zhenghaoz/gorse/cmd/goat	3.843s

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3528e93) 70.10% compared to head (846892a) 70.10%.

❗ Current head 846892a differs from pull request most recent head c36bfd5. Consider uploading reports for the commit c36bfd5 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #717   +/-   ##
=======================================
  Coverage   70.10%   70.10%           
=======================================
  Files          64       64           
  Lines       12238    12238           
=======================================
  Hits         8579     8579           
  Misses       2875     2875           
  Partials      784      784           

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