This is from Simon's suggestion:

* `tyCoVarsOfTypesAcc` is a terrible name for a function with a
  perfectly decent type `[Type] -> FV`. Maybe `tyCoFVsOfTypes`?
  Similarly others

* `runFVList` is also terrible, but also has a decent type.
  Maybe just `fvVarList` (and `fvVarSet` for `runFVSet`).

* `someVars` could be `mkFVs :: [Var] -> FV`.