Support being built with crypton rather than cryptonite

crypton is a fork of cryptonite, and cryptonite's github repo has been
archived. Some deps are already using cryptonite so it's clearly the way
forward.

Added a build flag without a default, so cabal configure will select on its
own which to use. stack files pin to cryptonite for now.

Sponsored-by: Nicholas Golder-Manning on Patreon

standalone/linux/stack-i386ancient.yaml

@@ -8,6 +8,7 @@ flags:
     dbus: false
     debuglocks: false
     benchmark: false
+    crypton: false
 packages:
 - '.'
 extra-deps: