As part of legacy/trac#24985, we'll need to implement ​prop#269. 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 2. It ultimately depends upon legacy/trac#24986 and legacy/trac#24987 as well, although it can be done (and even rolled out into production, if we chose to do so) before either are finished.