Run more of CI in parallel
Right now, the first full validate build during CI catches lots of different classes of errors, at least:
- warnings (via
- Haddock not building
- tests in the testsuite failing
Each of these can actually be tested independently of the others, yet they are done serially.
Could these be parallelized? That way, a contributor could get all the feedback at once, make changes, and then try again, instead of waiting a long time to get feedback by dribs and drabs.