Start getting superclass constructors working.

This also means the metaclass inheritance hierarchy parallels
the regular inheritance chain so that the subclass can find
the superclass constructor.

test/inheritance/inherit_methods.wren

@@ -2,12 +2,14 @@ class Foo {
   methodOnFoo { io.write("foo") }
   method(a) { io.write("foo") }
   method(a, b, c) { io.write("foo") }
+  override { io.write("foo") }
 }
 
 class Bar is Foo {
   methodOnBar { io.write("bar") }
   method(a, b) { io.write("bar") }
   method(a, b, c, d) { io.write("bar") }
+  override { io.write("bar") }
 }
 
 var bar = Bar.new
@@ -19,9 +21,6 @@ bar.method(1) // expect: foo
 bar.method(1, 2) // expect: bar
 bar.method(1, 2, 3) // expect: foo
 bar.method(1, 2, 3, 4) // expect: bar
+bar.override // expect: bar
 
-// TODO: Overriding.
-// TODO: Private fields.
-// TODO: Super (or inner) calls.
-// TODO: Grammar for what expressions can follow "is".
 // TODO: Prevent extending built-in types.