Move UTF8.h to UnicodeMapFuncs.h and rename UCS2 to UTF16

UTF8.h is not exclusively UTF-8 code. Renaming to UnicodeMapFuncs.h
identifies the file as containing maps for UnicodeMap and is
consistent with the name UnicodeMapTables.h.

The mapUCS2 code was changed to support UTF-16 in 979ef1ca without
changing the name.
3 files changed