Split varint into own package

[ed-flanagan]

I started this project just for fun -- practice some Elixir and explore Riot's deck code implementation. Since it was for fun, I rolled my own varint implementation, not looking for pre-existing libraries. However, the implementation is not project specific and feels out of place within this project (to me). Maybe it makes sense to split out into its own package?

The implementation, as is, is fairly generic, albeit lacking in some possible features/functions. Further, I don't want to litter the ecosystem with Yet Another library. Some other implementations I'm aware of:

• https://github.com/ahamez/varint -- dedicated library, seems to be somewhat active. If I feel something is missing, maybe a better idea just to contribute there (e.g., VLQ, signed values, etc.)
• https://github.com/elixir-protobuf/protobuf/blob/main/lib/protobuf/wire/varint.ex -- an internal implementation for Protobufs, but a popular one.