RSA-KEM support

[tarcieri]

RSA-KEM is a key encapsulation mechanism for transmitting a randomly generated cryptographic key (e.g. a symmetric key for a hybrid cryptosystem) between parties.

We have a trait-based abstraction over KEMs which provides a common API: https://github.com/RustCrypto/traits/tree/master/kem

It would be good to add (possibly optional/feature-gated) support for using the kem interface with RSA-KEM.

cc @rozbb