Improve error messages involving skolem variables

[byorgey]

There are two kinds of error messages we could get involving skolem variables.

• We can simply get a type mismatch when we try to unify a skolem variable with something else. However, the current error is confusing since it references a made-up variable. For example:

  > def f : a -> a = \x. 3 end
  1:22: Type mismatch:
    From context, expected `3` to have type `s1`,
    but it actually has type `Int`
  
    - While checking the definition of f
  

  It would be better if the error said something about polymorphism and mentioned the type variable a. This might require keeping track of a mapping between generated skolem variables and type variables in the polymorphic type.

• When a skolem variable escapes, the error message is even worse since the error explicitly mentions the word "skolem":

  > \f. (f:forall a. a -> a) 3
  1:5: Skolem variable s3 would escape its scope
  

  This case is more obscure and probably won't happen very much (especially once we fix Strange type checking error "Skolem variable would escape its scope" #2101), but it would still be nice to come up with a better error message that at least hints at what the problem might be.