utf8.c: Remove wrapper functions.

utf8.c: Remove wrapper functions.

Now that the Unicode data is stored in native character set order, it is
rare to need to work with the Unicode order.  Traditionally, the real
work was done in functions that worked with the Unicode order, and
wrapper functions (or macros) were used to translate to/from native.

There are two groups of functions: one that translates from code point
to UTF-8, and the other group goes the opposite direction.

This commit changes the base function that translates from UTF-8 to code
point to output native instead of Unicode.  Those extremely rare
instances where Unicode output is needed instead will have to hand-wrap
calls to this function with a translation macro, as now described in the
API pod.  Prior to this, it was the other way, the native was wrapped,
and the rare, strict Unicode wasn't.  This eliminates a layer of
function call overhead for a common case.

The base function that translates from code point to UTF-8 retains its
Unicode input, as that is more natural to process.  However, it is
de-emphasized in the pod, with the functionality description moved to
the pod for a native input wrapper function.  And, those wrappers are
now macros in all cases; previously there was function call overhead
sometimes.  (Equivalent exported functions are retained, however, for XS
code that uses the Perl_foo() form.)

I had hoped to rebase this commit, squashing it with an earlier commit
in this series, eliminating the use of a temporary function name change,
but the work involved turns out to be large, with no real payoff.