Implement a moving/compacting collector

[Others]

Since we have the whole GcHandle system, we have a nice way of figuring out if something can be moved or not. Could we exploit this to create a compacting collector somehow?

We'd need to be careful, because the worst case is probably someone holding a GcHandle forever. (Meaning we'd need to keep that data live.)

Note:

• This would prevent us allowing situational deref
• The perf win is probably not too much since we need to allocate for every handle anyway
• This should probably be done as part of a larger effort to make the backend pluggable