forest value should be u64 instead of usize (in MmrPeaks, Mmr, etc)

[plafer]

We currently use usize for the value of forest, which extends to many other places (such as MmrPeaks::num_leaves).

The size of usize is dependent on the hardware it's being run on. This is not exactly the behavior we want; abstractly, the VM can manipulate an Mmr with up to 2^64 - 2^32 leaves. Thus, a better choice would be to use u64, which converts naturally to/from a Felt.

It will also make the Mmr nicer to work with:

// before
let forest = Felt::from(mmr.forest() as u64);

// after
let forest: Felt = mmr.forest().into();

[bobbinth]

I would probably address this in the context of #196.