Crate genmap

TODO: Iteration! Fast iteration? TODO: We might be able to make it TRY to free memory, a la Vec::shrink_to_fit(), but making it possible to always free unused memory basically means implementing a copying GC. Even as is, shrink_to_fit() would require walking and rebuilding the whole freelist.

Structs

GenMap	A collection of T's referred to by Handle's. When you add an object to the GenMap it will return a Handle, and you can look that item up by that Handle. You can also remove the item, which makes any old Handle's to it become invalid and attempting to get it will return None.
Handle	A small, easy-to-copy handle referring to a location in a particular GenMap.
Iterator	Iterator over keys in a GenMap.