Better handling of UCNs in strings

Better handling of UCNs in strings

As the standard requires, take 4 hex digits after the \u opener of a
Universal Character Name, or take 8 hex digits after \U, but reject
smaller counts and don't consume more (https://port70.net/~nsz/c/c11/n1570.html#6.4.3,
https://port70.net/~nsz/c/c99/n1256.html#6.4.3).

The unicode codepoint used to get truncated to 1 byte. Now it gets expanded into UTF-8,
matching gcc & clang behavior on Linux.

TODO: Universal character names should also be supported in identifiers,
as in, e.g., char \u010dau_sv\u011bte[]="čau_světe";