Introduce case_character function

Introduce case_character function

Move single-character casing logic into a separate function so that
it is collected in a single place.  This will make future changes to
the logic easier.  This commit introduces no functionality changes.

* src/casefiddle.c (struct casing_context, prepare_casing_context): New
sturcture for saving casing context and function to initialise it.
(case_character): New function which cases character base on provided
context.
(do_casify_integer, do_casify_multibyte_string,
do_casify_unibyte_string, casify_object, casify_region): Convert to
use casing_context and case_character.