Revert recent GC-related changes (Bug#43152)

Revert recent GC-related changes (Bug#43152)

* src/alloc.c (live_string_holding, live_cons_holding)
(live_symbol_holding, live_large_vector_holding)
(live_small_vector_holding):
Go back to old approach of treating every would-be pointer to any
byte in the object (though not to just past the object end) as
addressing the object.
(live_float_p): Require that the would-be float point
to the start of the Lisp_Float, and not anywhere else.
(live_vector_pointer, live_float_holding, mark_objects):
Remove.  All uses removed.
(mark_maybe_object, mark_maybe_objects):
Bring back these functions.
* src/lisp.h (SAFE_ALLOCA_LISP_EXTRA): Do not clear the
new slots, as they're now checked via mark_maybe_objects,
not via mark_objects.