Investigate optimized utf8 decoders and validators

[thejoshwolfe]

inspired by some comments in #2099 , check out these algorithms for UTF-8 processing:

• https://github.com/cyb70289/utf8/
• https://lemire.me/blog/2018/10/19/validating-utf-8-bytes-using-only-0-45-cycles-per-byte-avx-edition/
• http://bjoern.hoehrmann.de/utf-8/decoder/dfa/

So far I'm concerned that none of the above properly validate UTF-8. None of them explain which validation checks they're doing, and Wikipedia lists several checks that are commonly overlooked. And because the above implementations are optimized, it's difficult to know what they're doing without testing, which is part of the objective for this issue.

In addition to nonsensical byte sequences, we also need to be sure to reject:

• Overlong encoding
• Surrogate half
• Overflow

We already have tests for these in the unicode.zig library (search for testError). Switching to an optimized implementation should not regress those tests.

[shawnl]

Looking at the atomic state graph on this page: http://bjoern.hoehrmann.de/utf-8/decoder/dfa/
and comparing it with this chart:

 * http://www.unicode.org/versions/Unicode6.0.0/ch03.pdf - page 94
 *
 * Table 3-7. Well-Formed UTF-8 Byte Sequences
 *
 * +--------------------+------------+-------------+------------+-------------+
 * | Code Points        | First Byte | Second Byte | Third Byte | Fourth Byte |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+0000..U+007F     | 00..7F     |             |            |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+0080..U+07FF     | C2..DF     | 80..BF      |            |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+0800..U+0FFF     | E0         | A0..BF      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+1000..U+CFFF     | E1..EC     | 80..BF      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+D000..U+D7FF     | ED         | 80..9F      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+E000..U+FFFF     | EE..EF     | 80..BF      | 80..BF     |             |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+10000..U+3FFFF   | F0         | 90..BF      | 80..BF     | 80..BF      |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+40000..U+FFFFF   | F1..F3     | 80..BF      | 80..BF     | 80..BF      |
 * +--------------------+------------+-------------+------------+-------------+
 * | U+100000..U+10FFFF | F4         | 80..8F      | 80..BF     | 80..BF      |
 * +--------------------+------------+-------------+------------+-------------+

They are identical.

[matu3ba]

1. Notable source for cross-validation libgrapheme from suckless project.
2. If SIMD is acceptable: simdutf8 or a port.
3. Article how-quickly-can-you-check-that-a-string-is-valid-unicode-utf-8 for own implementation.
4. Stuff for Benchmarking utf8