Move Python utilities into a separate Zcash library

[dannywillems]

I like we provide methods in Python, like a "spec". What do you think about exporting these primitives in a different packages?

Ethereum does that, and IMHO, it is a good approach for simplicity and readibility of protocols/primitives.

Originally posted by @dannywillems in #82 (comment)

[linear]

COR-1334