Support post-quantum key exchange

[DemiMarie]

DJB recommends NTRU Prime, but other algorithms are worth considering too. For maximum robustness, we might want to take a bunch of different algorithms and hash the results.

[saolof]

After the NIST standardization, CRYSTALS looks like the best option for general purpose, with the two other algorithms being useful for specific niches: https://pq-crystals.org/

[nashley]

Link to NIST publication: https://csrc.nist.gov/Projects/post-quantum-cryptography/selected-algorithms-2022

[briansmith]

Closing as a duplicate of #1685.