Simplify GC statistics-gathering

Simplify GC statistics-gathering

* src/alloc.c (make_interval, allocate_string, make_float)
(free_cons, Fcons, setup_on_free_list)
(allocate_vector_from_block, Fmake_symbol):
Do not update gcstat, since it is for statistics from the most
recent GC, not for a partially-updated hodgepodge.
(sweep_vectors): Update gcstat, since setup_on_free_list
no longer does.
(garbage_collect_1): Rename to garbage_collect and adopt its API.
Remove the old garbage_collect, which is no longer needed.
All callers changed.