Unique TypeVariables

[gardspirito]

Ensures there aren't any conflicting type variables.

Closes #64.

[gardspirito]

This PR does not perform necessary substitutions for check phase:

>>> let test : (forall (x : Type). x -> x) = let test2 : (forall (x : Type). x -> x) = \x -> let y : x = x in y in test2 in test
Not a subtype

The following type:

  x0

(input):1:63: 
  │
1 │ let test : (forall (x : Type). x -> x) = let test2 : (forall (x : Type). x -> x) = \x -> let y : x = x in y in test2 in test
  │                                                               ↑

… cannot be a subtype of:

  x

(input):1:98: 
  │
1 │ let test : (forall (x : Type). x -> x) = let test2 : (forall (x : Type). x -> x) = \x -> let y : x = x in y in test2 in test
  │                                                                                                  ↑

I guess there must be a better approach.