As part of legacy/trac#24985, we'll need to implement [prop#269](https://gitweb.torproject.org/torspec.git/tree/proposals/269-hybrid-handshake.txt). This should probably be in two parts: 1) Implement a new module which, given function pointers to two handshakes (one classic and the other post-quantum secure) which follow some prescribed API, compose the handshakes together to produce a final shared secret 2) Implement "hybrid null" (as it's called in the proposal), which here I'm calling "ntor2" This ticket is about part 1. It depends upon legacy/trac#24986 as well.