Add Fluent API (#251)

* Fully qualify types from models

For a long time, we've been fully qualifying depdency types but not model types. This produces code that's fine, but it's always been a confusing inconsistency and it posed problems trying to render Opaque types that specified namespaces (because the namespace was ignored).

This removes that inconsistency.

* A few more small refactorings

* Fix test failures

* Wip fluent API

* Fluent builders working!

* fix handle visibility

* Delete unused imports

* Lots of refactorings & making the fluent client an optional feature

* Cleanup module handling & add support for Cargo features

* Fix AWS tests

* Remove unused modules customization

* Set optional in the Cargo toml

* .execute() -> .send()

* Fix issues from refactoring out cargo features implementation