std: Speed up str::is_utf8

[bluss]

Use unchecked vec indexing since the vector bounds are checked by the
loop. Iterators are not easy to use in this case since we skip 1-4 bytes
each lap. This part of the commit speeds up is_utf8 for ASCII input.

Check codepoint ranges by checking the byte ranges manually instead of
computing a full decoding for multibyte encodings. This is easy to read
and corresponds to the UTF-8 syntax in the RFC.

No changes to what we accept. A comment notes that surrogate halves are
accepted.

Before:

test str::bench::is_utf8_100_ascii ... bench: 165 ns/iter (+/- 3)
test str::bench::is_utf8_100_multibyte ... bench: 218 ns/iter (+/- 5)

After:
test str::bench::is_utf8_100_ascii ... bench: 130 ns/iter (+/- 1)
test str::bench::is_utf8_100_multibyte ... bench: 156 ns/iter (+/- 3)

An improvement upon the previous pull #8133

[bluss]

Not addressing surrogate halves, only performance. With the code laid out in the new way, it's easy to see how we can disallow surrogates if wanted.

[brson]

Thanks for putting benchmark numbers in the commit message and pr.

[bluss]

thanks