Fix specs to use some helper functions

This is a lot more readable than having all of the explicit references
to the test vector class and encoders.  It's also less typing, and the
helpers can handle coverting to raw bytes.
Expose useful information through the API

There are now APIs for nonce_bytes, cryptographic primitive, signature
bytes, key bytes etc.  There are also more specific constants provided,
many of which are aliased to appropriate constants on the class, or
exposed through methods.
Using Public and PrivateKeys for the Box class

This centralises length checking, and also means that we can check for
use of the correct primitive!
Fix typo in specs

That passed with no problems on 2.0
Primitive information for authenticator classes

They should have it too, for simplicity and consistency.