core/heap.disabled: invoke underlying allocator directly

core/heap.disabled: invoke underlying allocator directly

The heap object stores its allocator for its entire lifetime, so
there's no need to call mowgli_alloc()/mowgli_free(), which also
stores the allocator tag behind the returned pointer.

This will (sometimes) reduce the memory consumption of every
allocation by a pointer and is slightly more efficient.

Also add some debugging assertions to validate the integrity of
the given/chosen allocator.