legacy: remove extraneous types
add 'fromKeyContent' function to Key class

Use cases have emerged where people want to construct JWK data from
raw key material (e.g., a ByteString for a symmetric key).  Add the
'fromKeyContent' function to the class, for this use case.
export JWA.JWK via JWK

Some library users need access to the JWA.JWK types and
constructors, so export them, via JWK (JWA.JWK remains hidden).