charnames: Don't return UTF-8 unless have to.

charnames: Don't return UTF-8 unless have to.

A Latin-1 range character doesn't require UTF-8 to be represented.  Not
using UTF-8 speeds things up a lot.  This commit changes the return to
be UTF-8 only if necessary.  This could be because
 1) the character isn't representable except in UTF-8; or
 2) we are being called at runtime and the character isn't ASCII-range.
    The only way we can currently guarantee that an upper-Latin1
    character will be treated with Unicode rules is to make it UTF-8
    (the Unicode bug).  The compile-time code (in toke.c) knows how to
    properly deal with non-UTF-8 upper Latin1 range characters.

The utf8::downgrade introduced in this commit is temporary.  Future
commits will change things so that there is no need for it.