JWA: extract JWK parameters to own module JWA.JWK
JWA.JWK: add JWK Parameters for Symmetric Keys
JWA: extract JWS parameters to own module JWA.JWS
JWA: extract JWE parameters to own module JWA.JWE
JWK: move Alg sum type to JWK

The Alg sum type defined in JWA is really defined in §3.3. of JWK,
so move it there (this will also help avoid circular imports).
JWA: remove now-empty JWA module