Refactor _Wikitext_utf32_char_to_entity (append to buffer)

Refactor _Wikitext_utf32_char_to_entity (append to buffer)

Rename the _Wikitext_utf32_char_to_entity function to
_Wikitext_append_entity_from_utf32_char, teaching it to
append to a target buffer directly rather than creating
a temporary Ruby String instance.

I don't particularly like these low-level manipulations but
the main goal here is to avoid the extra allocation; a
subsequent commit will clean up.

Signed-off-by: Wincent Colaiuta <win@wincent.com>