Methods for serialising and deserialising Nat to/from bytes

[ ] convert Nat object to sequence of bytes
[ ] convert sequence of bytes to Nat object
Note: these methods should be documented as portable between compatible versions of arby and across different platforms (i.e. you can safely send output of object bytes from v0.4.0 running on macOS to v0.4.5 on Windows and get the same number value out at the other end).
To be able to do this at compile-time, requires a dynamic-size container type like that used for our _digits array in Nat. This could be reused, of course...
- A neater way could be to create a proxy-object that just wraps _digits in a way that access into the proxy object is indexed to the individual bytes of the digits, without additional heap allocation. This could be a better way of doing it, really. We could even convert bytes back to object via such a technique!
We will encode/decode the value as big-endian sequences of bytes, and we will document it as such

saxbophone / arby