Move from cryptonite to crypton

[ocramz]

Apparently 'cryptonite' is not actively maintained, and 'crypton' is the new standard.

A number of key networking libraries have switched to 'crypton' already : https://packdeps.haskellers.com/reverse/crypton

It's API-compatible, so this should be just an import renaming chore.

[bmillwood]

• I attempted to do this and described the main blocker in Build with crypton #8

[puffnfresh]

Added a flag to do so: 2b3167c

I made it a flag since you already need a flag because of cryptostore. If ocheron/cryptostore#13 gets resolved, I'll happily remove the flag.