API Redesign & Ergonomics

The current API emulates the NIST code to follow along with the standardisation process. Now that is all done we can do a proper cleanup to make it more idiomatic along with general quality of life improvements. A non-comprehensive list of to-do's:

[ ] Keypair with no visible fields
[ ] Remove redundant public key field from Keypair
[x] Runtime public key function to extract from secret key
[x] Generate new Keypair struct from existing private key
[x] Generate new Keypair struct from 64 byte seed
[ ] Encapsulation/Decapsulation function arguments accepting impl AsRef<PublicKey> / impl AsRef<SecretKey>
[ ] Keypair - impl Hash - For public key only, non-cryptographic hashes often used
[ ] Keypair - impl Debug - For public key only, elide secret key from any possible logging
[ ] Keypair - impl PartialEq - For public key only, make note in docs that it is not constant time
[ ] Keypair - impl Eq
[ ] Keypair - impl Zeroize - will require an alternative api using Pin/ Box along with documentation. Needs a locally instantiated SecretBuffer by the end user to prevent return value optimisations copying everything willynilly.
[ ] Replace type alias for SharedSecret with a concrete type, implement the same traits as for Keypair above.

Related to #59, it's preferable to get this all wrapped up into one release.

Argyle-Software / kyber

API Redesign & Ergonomics #68