Support for generics or 64-bit leaf_index for Bvh

From what I've seen, `leaf_index` keys (used for `insert`, `remove`, etc with `Bvh`) are current `u32`. However, `leaf_node_indices` is using `usize` keys, which will be 64-bit on 64-bit systems.

My problem lies in having data in a `SlotMap` with 64-bit keys, and wanting to perform collision detection on that. It would be trivial to do if either the `leaf_index` was generic (or at least 64-bit). However, since I only care about 64-bit support, I would be happy enough to have it take a `usize` instead of a `u32` and just avoid the casting internally. If not, I'll have to keep a secondary table to map the collision indices to the `SlotMap` keys adding significant overhead to the process.

A similar (but much older) thing is discussed in #117 , which discusses making it generic over keys of `Eq + Ord + Hash`, which I don't think is necessary to be so strict as internally `Bvh` is using `VecMap`, however, using `HashMap` may be faster.