Fixes #37. Lookup Constants in both static and instance scopes.

[faultyserver]

References to Constants can be done from both static and instance scopes. Until now, the lookup was always naively being performed on the current_scope, which could be either static or instance, but both would not be checked.

This led to the bug described in #37, where the current_scope was the scope of an instance, while the constant was defined on the static scope of current_self. In this case, the constant should have been resolved, but wasn't because only the instance scope was being checked.

An example of this behavior is

deftype Foo
  deftype Bar
  end

  def make_bar
    %Bar{}
  end
end

# `instance` should become an instance of `Foo.Bar`.
instance  = %Foo{}.make_bar

An interesting observation is that if Bar was defined outside of Foo, this bug would not occur, because both static and instance scopes have their parent set to the lexical scope they are defined in, and in that case Bar would have been defined in that lexical scope. In this case, however, Bar is defined on the static scope of Foo.

This behavior of lexical scoping might change in the future, where instance scopes won't see their lexical scope, but shouldn't cause issues beyond this.