Docs.rs + README changes for 2.x

[pinkforest]

Rendered

• The structure follows more curve25519

Assumes/TODO:

• Get rid of rand in favor of optional rand_core
• Optional digest and zeroize

Merged doc changes from:

• Make verify_batch deterministic #256

[rozbb]

There's a lot of edits I'd like to do here and the CHANGELOG. Merging this now, and gonna do a follow-up PR