fix(codegen): Fix bug triggered by operation input/output naming (#699)

* fix(codegen): Fix bug triggered by operation input/output naming

During codegeneration, synthetic copies of input & output shapes are created. However, due to a Smithy shape id conflict, these
shapes were overwriting existing shapes in the model instead of creating new shapes.

This lead to a number of bugs exposed by the new s3control model. A minimal example of this model is included as a test in rest-xml-extras.

This commit augments the namespace of synthetic shapes to exist in the `synthetic` Smithy namespace, removing the conflict. After this bug was fixed, a subsequent bug in the serializer function naming was exposed where two shapes with the same name but in different modules generated a conflicting serializer.

Tests:
- [x] new s3control model compiles
- [x] test case added to rest-xml-extras

* cl: update changelog

* fix(codegen): Fix smithy-rs#662 & cleanup

* Update CHANGELOG.md

* fix another unit test