Library for counting length of chars faster than Chars::count()

Idea is from UTF-8のコードポイントはどうやって高速に数えるか, and UTF-8のコードポイントはどうやってもっと高速に数えるか.

usage

//before
"Hello, world!".chars().count();

//after
"Hello, world!".chars_count();

bench

repeated "a" (only 1byte utf8)

Length	std	chars_count	chars_count(avx2)
1	1.3906 ns	3.1607 ns	2.1750 ns
10	4.7844 ns	5.6093 ns	7.2465 ns
100	47.630 ns	10.607 ns	11.265 ns
1000	475.17 ns	67.364 ns	17.839 ns
10000	4.7231 us	604.66 ns	91.757 ns

repeated "錆" (only 3byte utf8)

Length	std	chars_count	chars_count(avx2)
1	2.4324 ns	4.5069 ns	4.2662 ns
10	13.947 ns	10.267 ns	11.984 ns
100	143.74 ns	23.935 ns	28.758 ns
1000	1.4210 us	193.98 ns	40.435 ns
10000	1.8505 us	270.60 ns	14.206 us

See performance branch for details.

future plan

sse (128bit)

avx512 UTF-8のコードポイントはどうやってAVX-512で高速に数えるか