Split casify_object into multiple functions

Split casify_object into multiple functions

casify_object had three major cases to cover and those were mostly
independent of each other.  Move those branches to separate function
so it’s easier to comprehend each individual case.

While at it, use somewhat more descriptive ch and cased variable names
rather than c and c1.

This commit introduces no functional changes.

* src/casefiddle.c (casify_object): Split into…
(do_casify_integer, do_casify_multibyte_string,
do_casify_unibyte_string): …new functions.