Speed up/reduce allocations by using strings.Map

[artyom]

With the following benchmark:

var sink string

func BenchmarkCreate(b *testing.B) {
	b.Run("Simple", func(b *testing.B) {
		const text = `Strings, bytes, runes and characters in Go`
		b.ReportAllocs()
		b.SetBytes(int64(len(text)))
		for i := 0; i < b.N; i++ {
			sink = sanitized_anchor_name.Create(text)
		}
	})
	b.Run("Complex", func(b *testing.B) {
		const text = `日本語; «Ёжик в тумане»?!`
		b.ReportAllocs()
		b.SetBytes(int64(len(text)))
		for i := 0; i < b.N; i++ {
			sink = sanitized_anchor_name.Create(text)
		}
	})
}

The results are:

name              old time/op    new time/op     delta
Create/Simple-8      434ns ± 0%      187ns ± 1%   -56.99%  (p=0.000 n=10+10)
Create/Complex-8     667ns ± 0%      579ns ± 0%   -13.26%  (p=0.000 n=10+10)

name              old speed      new speed       delta
Create/Simple-8   96.8MB/s ± 0%  225.0MB/s ± 1%  +132.53%  (p=0.000 n=10+10)
Create/Complex-8  61.5MB/s ± 0%   70.9MB/s ± 0%   +15.30%  (p=0.000 n=10+10)

name              old alloc/op   new alloc/op    delta
Create/Simple-8       552B ± 0%        48B ± 0%   -91.30%  (p=0.000 n=10+10)
Create/Complex-8      296B ± 0%        48B ± 0%   -83.78%  (p=0.000 n=10+10)

name              old allocs/op  new allocs/op   delta
Create/Simple-8       7.00 ± 0%       1.00 ± 0%   -85.71%  (p=0.000 n=10+10)
Create/Complex-8      6.00 ± 0%       1.00 ± 0%   -83.33%  (p=0.000 n=10+10)