Tlsa/ucs4 normalize (#88)

Tlsa/ucs4 normalize (#88)

* Split codepoint sequence normalisation out into separate function.

This creates utf8proc_normalize_utf32() which takes and returns
a UTF-32 string, applying the following options:

- UTF8PROC_NLF2LS
- UTF8PROC_NLF2PS
- UTF8PROC_NLF2LF
- UTF8PROC_STRIPCC
- UTF8PROC_COMPOSE
- UTF8PROC_STABLE

The utf8proc_reencode() function has been updated to call the
new utf8proc_normalize_utf32().

* Update code documentation: utf8proc_reencode handles UTF8PROC_CHARBOUND.