Support this pseudo-params in classes, and enforce subtyping on structural class statics

[popham]

This patch adds support for a this pseudo-param on class methods (static and non-static). With the syntax someMethod(this: this & I,... users can impose requirement beyond "is subtype of the enclosing class," the current behavior. Some goofiness becomes admissible too, e.g. someMethod(this: {})--just don't be goofy (maybe a warning is warranted if the pseudo-param's type is not a subtype of the enclosing class). I have a use case where the value in this extension lies in chaining of static methods, not non-statics, so I've extended class subtyping to structural class statics (InstanceT ~> SuperT still ignores subtyping of statics). I feel like I'm on solid ground after a chat with @jeffmo and @avikchaudhuri on IRC, although during that discussion I forgot to contrast ES5 and Node inheritance against ES6 inheritance. I recall in the past that @samwgoldman and I disagreed on whether class statics should subtype.

Node's inheritance (util.inherit) and the ad hoc inheritance of ES5 (SomeClass.prototype = Object.create(ParentClass.prototype, {...}) without SomeClass.proto = ParentClass) do not chain class statics. Subtyping statics is biased toward ES6 inheritance, whereas the status quo is biased toward Node and ad hoc inheritance.

Closes #807, #1369, #1704, #1705

[popham]

New PR for rebase