I change the commit message to reflect that. The story is that it was not clear before if a `uint` is an index/len or if it is the machine representation used internally. That's why I used `MachineUint` to be clear.

I do not use directly u64 for future optimization. For example, I can imagine that when we have `u128`, MachineUint cat be `u128` if the performances are better.

Finally, maybe the cleanest thing to do is to make `BigDigit` private because it looks like internal plumbing, not public API.