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)
std: convert first_non_utf8_byte to use the iterator.

This makes it very slightly faster, especially when the string is valid
UTF-8, and completely removes the use of `unsafe` from the first half.

Before:

    from_utf8_lossy_100_ascii              ... bench:       151 ns/iter (+/- 17)
    from_utf8_lossy_100_invalid            ... bench:       447 ns/iter (+/- 33)
    from_utf8_lossy_100_multibyte          ... bench:       135 ns/iter (+/- 4)
    from_utf8_lossy_invalid                ... bench:       124 ns/iter (+/- 10

After:

    from_utf8_lossy_100_ascii              ... bench:       119 ns/iter (+/- 8)
    from_utf8_lossy_100_invalid            ... bench:       454 ns/iter (+/- 16)
    from_utf8_lossy_100_multibyte          ... bench:       116 ns/iter (+/- 9)
    from_utf8_lossy_invalid                ... bench:       119 ns/iter (+/- 9)