Define and use Basic Types for Public Key and Signature

[brendontoken]

This was prompted by the opportunity to change the E1 - Order field AuthorityPublicKey from a variable length field, into a fixed length field, to benefit the protocol by being less ambiguous. The AuthorityPublicKey field also has a associated AuthorityIncluded field to determine when it should be included, so no need to make the field variable length for that purpose.

The Signature field can have more constraints than a purely variable length field.

These new basic types should be documented and used where appropriate in the different actions.