Start a Unicode library implementation

[mcy]

This change adds:

• String manipulation operations to best::str.
• Basic properties to best::rune.
• Fixes bugs in best::utf8::encode().
• Adds tests for all of the above.

We also added bytes.h, which are convenient wrappers over the mem* functions. We also deleted encoding.h and dropped support for stateful encodings. Non-synchronizing encodings are still supported, but under some more constrained hypotheses.