Improve variable names

[MaximilianAlgehed]

Currently we do x_0, y_1, etc. in the pretty-printer. This is to ensure unique names and avoid overlap. We should only print the _# suffix when there is actual overlap.

[MaximilianAlgehed]

This code relies on this function:

findNameHint :: HasVariables t => Var a -> t -> Var a
findNameHint v t =
  case [nameHint v' | Name v' <- Set.toList $ freeVarSet t, nameOf v' == nameOf v, nameHint v' /= "v"] of
    [] -> v
    nh : _ -> v {nameHint = nh}

Two issues:

1. It's local to the current scope, so if x appears twice in two different places as a name hint, we can't tell here
2. The use of this function causes (at least) quadratic blowup.

The fix for this issue is probably to rewrite the name-hint code from the ground up, using a different approach to what we use now. You'd think you can hack around this by renaming only inner uses of x to x_#, but we really want all duplicated variables to have _# suffixes otherwise things risk becoming super confusing when looking at solver plans.