Switch to using `BoxedUint`?

Having to manage four different `Uint` sizes is not very convenient. With some recent changes in `crypto-bigint` and `crypto-primes`, it seems that the performance of `BoxedUint` is close to that of `Uint`. Should we switch? I expect there will be some performance regression, my estimate is 5-10%, but I don't know how to verify that without doing all the refactoring work. Would it be worth it?