Implement conversion traits based on integer size

[isaacholt100]

Current behaviour is that From<uN> is implemented for all sizes of BUint<N>, but if e.g. a. U80 was created (with the u8_digit Cargo feature enabled), then this would not be ideal, as From<u128> could fail, and so the conversion trait should be TryFrom instead.

This could be implemented using bounds on N, but unfortunately it would rely on the unstable and incomplete nightly feature generic_const_exprs. For now, I think that is the only solution.