Pool implementation

[nickik]

Current pool implementation is to complicated. A simple wrapper is all we need.

[gandro]

I think the Chunk should contain a pool ref, to ensure it does not outlive the pool (previously done using the 'pool lifetime).

[nickik]

I hope we can add this feature back in. It is more important then the automatic de-allocation.

My goal is to set up something simple that compiles so we can have some running tests for the pools. We can then implement these features one by one on top.

I can't deal with lifetime stuff on top of all the other problems I have with the Rust code.

[gandro]

Oh, I don't think we need (or even want) a livetime constraint on pools, but use refcouting instead. It's really not that complicated, instead of storing the raw pointer (mps_pool_t), you need to store a PoolRef.