Upgrade to dune 2

I want to upgrade to dune 2 to get support for extraction from Coq.

This does mean any future dependees on LIGO will be forced to upgrade to dune 2. Theoretically they can still get backwards-compatible behavior in their own project, I guess, so maybe not so bad.

The only change I have noticed is that (alias (name foo) ...) is now (rule (alias foo) ...), which is what is happening in this commit.