Strict conversion of UHC text to Unicode

Previously, mbstring would accept a lot of things which were not valid
UHC text. No more.

- Don't allow single-byte control characters t

Strict conversion of UHC text to Unicode

Previously, mbstring would accept a lot of things which were not valid
UHC text. No more.

- Don't allow single-byte control characters to appear where the 2nd
byte of a multi-byte character should be.
- Validate that the 2nd byte of a multi-byte character is in the
expected range.
- Treat it as an error if a multi-byte character is truncated.

Also add a test suite to confirm that UHC conversion (both to and from
Unicode) works according to spec.

show more ...