need backwards-compatile to_utf8_foo()

need backwards-compatile to_utf8_foo()

These 4 functions have been replaced by variants to_utf8_foo_flags(),
but for XS code that called the old ones in the Perl_to_utf8_foo()
forms, backwards compatibility versions need to be created.

For calls of just the to_utf8_foo() forms, macros have been used to
automatically call the new forms without the performance penalty of
going through the compatibility functions.