Cleanup around tree

[raphaelrobert]

This PR cleans up code around the ratcheting tree to correct previous design choices and make the transition to a full tree easier:

• Limits valid platforms to 32, 64, and 128 bits of pointer width
• Avoids unnecessary LibraryErrors in the binary tree
• Improves how leaves are returned, making the public MlsGroup.members() a nice iterator that's no longer a Result
• Improves merges, making MlsGroup.merge_staged_commit() LibraryError-free
• Deletes now unnecessary code

The PR is split into commits that can be reviewed separately. Each commit contains a small set of code changes and the corresponding churn.

Note that this is just a partial cleanup and that more needs to be done.

[kkohbrok]

I can't remember why we didn't just use config flags to restrict the architecture to begin with. I only remember that we discussed it at length and that I spent quite a bit of time to implement the current solution.

That being said, I think the solution in this PR is quite a bit more elegant than using library errors.

[kkohbrok]

I think getting rid of Itertools is nice, but loop is a bit dangerous. I seem to remember that we decided against using it in another instance due to the potential for an infinite loop, but I can't remember the specifics. This one seems pretty straight-forward due to how next is used in each iteration indicating that the loop must terminate at some point, so I don't have a strong opinion here.