2020-05-11: Cryptography design

Creates a new Primitives section for the cryptographic primitives Penumbra will use.

Adds a rationale for the choice of proof system.

Adds an overview of the decaf377 construction with an overview of the relative cost compared to other encodings, sketches the encoding and decoding functions and adds test vectors. Specifying the square root algorithm and hash-to-group map go together.

Also puts in placeholders for a Poseidon instance, and a STROBE-with-Poseidon construction.