Offer conversion with `Natural`.

A task one sometimes has to deal with is to take a number and look at the bytes it is made of. In other words, a number can be seen as an array of bytes. Specifically, there is a one to one inlay of numbers into byte strings and its inverse from byte strings back to numbers. It would be convenient if such a pair of functions could be found in this package.

There are two ways to encode a number into an array: least or most significant bit first. We can pick whichever is faster. There is also some confusion as to whether the encoding of 0 should be an empty byte string or a lonely zero byte. Neither is more mathematically principled than the other — we can pick whichever is more handy for our algorithm.

I think there is a linear time algorithm for either most significant or least significant digit first. But we can talk about implementation details once the design is agreed upon.