Refactor the wast resolver

The purpose of this commit is to cut down on the complexity of reading
and understanding the resolver for components. These were some
refactorings I noticed during review that I think help make it a bit
more clear in each function what's happenign.

* Add a new `Resolver` type to encapsulate the `Vec<ComponentResolver>`
  stack
* Move freestanding functions to methods on `Resolver` and remove
  lifetime arguments on each function.
* Rename the `ComponentResolver` type to `ComponentState` to avoid
  conflicting with `Resolver`.
* Move `register*` functions to `ComponentState`