std::str: safen and optimize is_utf8.

This uses a vector iterator to avoid the necessity for unsafe indexing,
and makes this function slightly faster. Unfortunately #11751 means that
the iterator comes with repeated `null` checks which means the
pure-ASCII case still has room for significant improvement (and the
other cases too, but it's most significant for just ASCII).

Before:

    is_utf8_100_ascii             ... bench:       143 ns/iter (+/- 6)
    is_utf8_100_multibyte         ... bench:       134 ns/iter (+/- 4)

After:

    is_utf8_100_ascii             ... bench:       123 ns/iter (+/- 4)
    is_utf8_100_multibyte         ... bench:       115 ns/iter (+/- 5)

0 files changed, 0 insertions, 0 deletions