Improve performance of status map parser

[itchyny]

This PR proposes improvement of #20. The new implementation reduces allocations as possible, and about 30% faster. Also this way of parsing is more Go-way.

func BenchmarkParse(b *testing.B) {
	for i := 0; i < b.N; i++ {
		Parse("unknown=critical,warning=critical")
	}
}

name     old time/op    new time/op    delta
Parse-8     429ns ± 0%     296ns ± 1%  -31.06%  (p=0.000 n=9+10)

name     old alloc/op   new alloc/op   delta
Parse-8      256B ± 0%      240B ± 0%   -6.25%  (p=0.000 n=10+10)

name     old allocs/op  new allocs/op  delta
Parse-8      7.00 ± 0%      3.00 ± 0%  -57.14%  (p=0.000 n=10+10)

[lufia]

strings.CutPrefix is added in Go 1.20.

Note: We will need to update workflows repo.

[coveralls]

coverage: 85.185% (+2.9%) from 82.278% when pulling 78b3761 on itchyny:improve-as-status into cea26f1 on mackerelio:support-as-status.

[lufia]

@itchyny thanks!

... CIs are still broken because Coveralls rejects the uploads when the coverage uses same build-id to any one of past builds.

Unfortunately we cannot delete the build from Coveralls, so I'm going to merge this to base branch, then runs CIs in that.