flisp: Fix memory leaks
There's two independent issues here:
1. The table allocator assumes that small tables will be stored inline
and do not need a finalizer. This is mostly true, except that hash collisions
can cause premature growing of the inline table, so even for relatively small
tables, we need to validate that the storage was not allocated out-of-line.
2. It is unsafe to clear the vtable finalizer pointer during the table allocation
to supress the `add_finalizer` call. This is because the allocation of the table
object itself may trigger GC of a different table, and without the finalizer set
in the vtable, freeing of that table's memory space would get skipped.
