memorypool: make rc's api like std

[jkarneges]

This renames the constructor from new to try_new_in, and impls Deref and Clone, to match how std's API works. This does a few things for us:

• Frees up new which we can reintroduce after this lands for using the system allocator instead of a memory pool. Having a single Rc type able to work with both kinds of memory sources is the optimal approach for supporting both in our linked list nodes (as opposed to wrapping two different Rc types in an enum and using that for links).
• Once we have both new and try_new_in, our Rc basically becomes a mirror of std while we wait for the allocator API to stabilize.
• Tracking std makes it easier to swap between our type and std, for benchmarks or possible removal.