Convert some varray stacks into vector stacks

[Nathan Sidwell]

Diego Novillo wrote:
> I was going to start changing more varrays, but there's an interface
> wrinkle in VEC_* that I'd like to address first.  The way we tell VEC_*
> that a vector should be allocated in the heap is pretty intrusive.  If
> we have a type T which we sometimes want on the heap and other times on
> GC memory, we need to create a separate type T' first.  This happened to
> 'tree'.  We are now forced to have a second type 'tree_on_heap' for
> vectors we want in the heap, because 'tree' is already tagged for GC
> vectors.

It appears that design decision wasn't so good.  I did not expect
such dynamism in the allocation choice :(

> Perhaps we could change that when we allocate the vector the first time?
> Something like 'vec = VEC_alloc (T, size, HEAP)'.
we'd also have to amend VEC_safe_push, VEC_safe_insert and VEC_reserve.

The alternative of holding state in the VEC to indicate its allocation
scheme (like the bitmaps), won't work, because an empty vector can
be represented with a NULL pointer.

Of course this would be 4.1 material, so we don't have to go rushing into
things.

nathan

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