Suppress GC stats when obviously not needed

Suppress GC stats when obviously not needed

This should help future improvements where these stats can be
bignums that do not fit into intmax_t.
* src/alloc.c (struct gcstat, gcstat): New type and static var,
to package up GC statistics into one C object.  It replaces ...
(total_free_intervals, total_intervals, total_strings)
(total_free_strings, total_string_bytes, total_vectors)
(total_vector_slots, total_free_vector_slots): ... these
removed static vars.  All uses changed.
(garbage_collect_1): Accept a struct gcstat *, not a void *
which was not used anymore anyway.  Return a bool indicating
success, instead of a Lisp object.  All callers changed.
(garbage_collect): New function.  All C callers of
Fgarbage_collect changed to use it, since none of them use the
return value.  Now, only Lisp code uses Fgarbage_collect.
(Fgarbage_collect): No longer noinline.  Cons up the return
value here, not in garbage_collect_1.