Ongoing work on multiple pages

[jorendorff]

#4 is fixed but there's a bunch of stuff I didn't have time for.

To test:

• [ ] allocating >1 page works
• [ ] pointers from one page to another keep referents alive
• [ ] gc collects cycles across page boundaries
• [ ] page limit >1 works (try_alloc returns None)
• [ ] hitting the limit, raising it, and hitting it again works
• [ ] allocating >1 page, setting limit of 1 page, things don't explode
• [ ] after collecting an object on one page, in non-fifo order, allocation finds and uses the gap

To do:

• [ ] use linked lists of pages and a freelist, instead of vec
• [x] move sweep_fn to the pageset

[jorendorff]

0d75da9 changed PageSet to use a linked list instead of a Vec<PageBox>.