Port hexdigit init to non-GCC + pdumper

Port hexdigit init to non-GCC + pdumper

The old code assumed that hexdigit initialization (needed by
non-GCC) could be done in syms_of_character, but that is no longer
true with pdumper.  Instead, simplify hexdigit init so that it can
be done statically on all C99 platforms.  Problem discovered on
Solaris 10 sparc + Oracle Solaris Studio 12.6.
* src/character.c (hexdigit): Add 1 to every value; all uses
changed.  This simplifies the initialization so that it can be
done statically on any C99 compiler.  hexdigit is now always const.
(syms_of_character): Omit no-longer-necessary initialization.
* src/character.h (HEXDIGIT_CONST, HEXDIGIT_IS_CONST):
Remove.  All uses removed.