Update with recent discussion from Luke

* The top-level item is now a "wit package"
* A package is a collection of documents, and documents are a file
* `use` can happen between interfaces of a file, between interfaces
  across documents in a package, and across packages
* Documents are always a flat list of interfaces/worlds/etc
* The binary encoding now is updated to accommodate restrictions for
  resources, this new flat list, and such.

Author: alexcrichton