Support for large objects

[pliniker]

Currently, the allocator only allows objects up to 32k (the Immix block size) - large objects are not supported.

The code path in stickyimmix/heap.rs:114 that raises an error should implement some other mechanism for tracking large objects.

I haven't thought this through yet but my initial half-baked thought has been to keep a list (linked list? Vec?) of large objects, where memory for large objects might be backed by Vec<u8> and the object header stored in the initial word(s).