JWA: update comment for JWA v16
types: add Base64Octets

Several fields specified by JWA and the other documents are
base64url encoded octet strings of arbitrary length.  Add spec for a
Base64Octets type (using the "iv" and "tag" examples from JWE) and
implement it.
JWA: implement §4.1. JWE "alg" parameter values

Implement JWA §4.1.  "alg" (Algorithm) Header Parameter Values for
JWE by way of the JWEAlgHeaderParameters sum type.