Optimize memory allocations on building patterns with strings.Builder

[rhysd]

This PR contains the following improvements

• Optimize building patterns with strings.Builder (the primary change)
  • strings.Builder was added at Go 1.10 (the version in go.mod is 1.12)
• Add BenchmarkGlob to measure performance
• Small fixes to make staticcheck happy
• Fix a badge in readme

The performance gain by this PR is as follows:

Before:

$ go test -bench . -benchmem
goos: darwin
goarch: amd64
pkg: github.com/mattn/go-zglob
cpu: Intel(R) Core(TM) i9-10910 CPU @ 3.60GHz
BenchmarkGlob-20             236           5027318 ns/op          249671 B/op       4139 allocs/op
PASS
ok      github.com/mattn/go-zglob       2.004s

After:

$ go test -bench . -benchmem
goos: darwin
goarch: amd64
pkg: github.com/mattn/go-zglob
cpu: Intel(R) Core(TM) i9-10910 CPU @ 3.60GHz
BenchmarkGlob-20             238           5011009 ns/op          249391 B/op       3706 allocs/op
PASS
ok      github.com/mattn/go-zglob       1.823s

Summary:

• Allocations: 11.68% smaller
• CPU: 0.325% faster (identical)

[mattn]

Thank you!