feat: Parallelise the type checker

[croyzor]

Branch inference-wip/fork is the current state of the art, which is to say, old enough we've forgotten much of it.
TODOs:

• rebase off the old holes branch to follow just-nat-solving (feat: Add nat hope solving #70)
• Parallelise typeEqs in SolveHoles.hs - note typeEqRow does the right thing by returning a bunch of unsolved/unexecuted subproblems, which the caller then parallelises via sequenceA