obstack: avoid potentially-nonportable function casts
* lib/obstack.c (CALL_CHUNKFUN, CALL_FREEFUN):
Remove, replacing with ...
(call_chunkfun, call_freefun): New static functions.
All uses changed. Avoid potentially-nonportable casts.
(chunkfun_type, freefun_type): Remove typedefs; no longer used.
(_obstack_begin_worker): Omit last two args, since they
rely on potentially-nonportable casts. All callers changed.
* lib/obstack.h (_OBSTACK_CAST): New macro.
Use it everywhere the old API used a potentially-nonportable cast.
The new API doesn't cast.
(struct obstack): Use unions rather than requiring
potentially-nonportable casts.
(obstack_chunkfun, obstack_freefun): Return void.