Add support for hybrid forward secrecy (HFS) extension

A draft of a hybrid forward secrecy (HFS) extension for the Noise protocol is available at https://github.com/noiseprotocol/noise_hfs_spec/blob/master/output/noise_hfs.pdf. Even though there's a lot that's up in the air, it might be fun to give it a try!