Document unsafe code

[tcharding]

We have a whole bunch of unsafe code and non of the safety preconditions are documented.

If #448 merges, remove the compiler attributes in main crate and also secp256k1-sys crate that allow the lint: #![allow(clippy::missing_safety_doc)].

[Kixunil]

Doing unsafe code review while documenting would be interesting for me. Reminds me #359 is still open.