Support multiple pages of objects (per type per heap)

[jorendorff]

Currently, we allow at most 1 page of objects per gc_ref_type! in your program, per Heap. The number of objects of any given type you can allocate, then, is roughly 4096 divided by the size of the type.

This is a pretty silly restriction, and it should be easy to lift. (The hardest part is retaining the ability to test full and near-full heap conditions.)