Make it possible to extract the underlying operations

[koute]

It'd be nice to be able to extract the exact shift/multiply/add/negate operations from the types here, instead of only being able to do arithmetic without the ability to peer inside.

One use case for this would be when you're writing a compiler and you want to emit code where the dividend is not known.

[dtrifuno]

I am considering eventually adding more algorithms, and having the crate select the best implementation for a given platform using cfg attributes, so I would rather have QuickDiv keep everything private for now. However, it'd probably make sense then to split it into two crates, one with the current interface, and the other containing the actual algorithms with everything fully public.