Support WebIDL inheritence

[fitzgen]

https://heycam.github.io/webidl/#dfn-inherit

Example:

interface A {
  void f();
  void g();
};

interface B : A {
  void f();
  void g(DOMString x);
};

Blockers:

• Need to figure out how to represent inheritance in general: Can we use traits to establish the inheritance hierarchy when importing JS classes and interfaces? #152

[alexcrichton]

Done in #640!