Refactor Operation shape builders (#68)

* Refactor structure builders out of StructureGenerator

* Refactor Operation Shape

This commit builds on the builder refactoring to introduce `Operation`s as the top level object that users interact with. This has a number of improvements:

1. The operations page in the documentation is now clean and includes exactly the operations that are available.
2. Builders for Input shapes now actually return an `Operation`. In a follow-up commit, the `build()` method on input shapes will be
updated to accept a service configuration object.
3. The split-impl block for operation shapes has been removed.
4. Docs for operation shapes are now the documentation for the operation and not for the input
5. This sets us up for having a top level operation onto which we can attach a `from_response(..)` method in a follow-up PR to implement response parsing.