Fix scoping and type checking of recursive functions

[pvdrz]

This PR fixes two bugs:

• When looking for the type of a variable, the type-checker was traversing the scopes in the reverse order, giving priority to shallower bindings. In a nutshell, the following program was considered correct:

  x: Int = 5
  x: Bool = 10
  x + 1

• The type-checker was not checking that the type of the body of a recursive function coincided with the annotated type in the signature of the function. Meaning that the following program was considered correct:

  fn foo(x: Int): Unit do
  foo(x)
  10
  end

Those cases were added as tests too.