Refactor typechecker to hide type substitution

[EliasC]

This commit refactors the way type substitution is handled so that it is
completely encapsulated in the lookup rules of the environment. As an
example, take the following class

passive class Foo<t>
  f : t

class Main
  def main() : void {
    let x = new Foo<int>;
    x.f = 42;
  }

In order to ensure that the assignment on the last line is valid, we
need to build a set of bindings that maps t to int, so that the
field f : t looks like f : int when we do the lookup. Instead of
cluttering the typechecker with this sort of thing, this translation is
now done implicitly on field lookup.

Since it's refactoring, I have included no new tests (or documentation).

[supercooldave]

Argh. I'll never the get type synonyms done if you keep changing things!!

[EliasC]

@supercooldave: I'm fine with postponing this merge until the type synonyms are done, and then the problem is mine to fix instead! My guess is that if anything this refactoring should make things simpler.

[supercooldave]

@EliasC I can see whether this causes any chaos – I was mainly moaning because I'd just managed to finish processing the last batch of changes to get type synonyms up to date.