Set up CI

[polybeandip]

This PR closes #43 by making the following changes

• move DSL tests from bin/main.ml to test/well_formed.ml
• set up CI according to issue #43 and @anshumanmohan's comment below
• fill in dune-project

[polybeandip]

I've set up a formatting check for CI: now, commits for which dune fmt is not a no-op will not pass CI.

Before we hit the big green button, would you mind confirming if this is the desired behavior?

[anshumanmohan]

Yes looks good to me, go ahead and merge when ready :)