Add support for interface mixins

* Obsolete use of [NoInterfaceObject] extended attribute as mixins.
* Add new interface mixin and partial interface mixin constructs.
* Replace implements statement by includes statement which only accepts
  mixins on its rhs.
* Remove supplemental interface and related concepts altogether.
* Add generic members dfn.
* Add table to clarify which members each construct accepts.
* Refactor default toJSON operation, and [Exposed] and [SecureContext]
  algorithms accordingly. Closes #118.
* Prevent operation overloading across mixins and interfaces. Closes #261.

Closes #363.
Closes #164.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=26452.
Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=25495.