utf8.c: Use existing macros instead of duplicate code

In all these cases, there is an already existing macro that does exactly
the same thing as the code that this commit replaces.  No sense
duplicating logic.
utf8.c: Move a bunch of deprecated fcns to mathoms.c

These functions will be out of the way in mathoms.  There were a few
that could not be moved, as-is, so I left them.