#5127: Even on narrow unicode builds, the C functions that access the Unicode

#5127: Even on narrow unicode builds, the C functions that access the Unicode
Database (Py_UNICODE_TOLOWER, Py_UNICODE_ISDECIMAL, and others) now accept
and return characters from the full Unicode range (Py_UCS4).

The differences from Python code are few:
- unicodedata.numeric(), unicodedata.decimal() and unicodedata.digit()
  now return the correct value for large code points
- repr() may consider more characters as printable.