begin separation of x509 crate from cryptography crate

[reaperhulk]

this will not be a published crate for now and the separation is incomplete. Putting up for commentary, although if we agree this is generally the right path we'll want to land this (+ review feedback) and then start tracking some tasks for improvement. The goal here is a crate that we may choose to publish that is useful to rust users and provides a clean separation of x509 parse (and encode?) logic from our pyo3 code.

[alex]

Plz don't rebase for ease of future reviews :-)

Does the coverage + cargo test stuff not need updating? (I think the cargo test stuff will just work if you set up the workspace)

[reaperhulk]

I've fixed coverage mostly -- there's one missed line on a derive statement that I don't understand but the other is probably real.

[reaperhulk]

One issue that I don't understand but otherwise full coverage now.