Adopt new KeyEncapsulation interface with RSA key exchange

[TrueSkrillor]

#153 changed the KeyEncapsulation interface to meet the needs of PQC key encapsulation schemes. However, the RSA key exchange (the only key encapsulation scheme from classical cryptography that SSH supports officially) implementation differs from the more general interface. This should be easy to fix (as of now all methods will throw NotImplementedException when called).