Add macros to allocate temporary Lisp objects with alloca.

Add macros to allocate temporary Lisp objects with alloca.
Respect MAX_ALLOCA and fall back to regular GC for large objects.
* character.h (parse_str_as_multibyte): Move prototype to ...
* lisp.h (parse_str_as_multibyte): ... here.
(struct Lisp_Cons): Add GCALIGNED attribute if supported.
(scoped_cons, scoped_list2, build_local_vector, build_local_string):
New macros.
(scoped_cons_init, pointer_valid_for_lisp_object, local_vector_init)
(local_string_init): New functions.
* alloc.c (verify_alloca) [ENABLE_CHECKING]: New function.
(init_alloc_once): Call it.