* [RemoveUnusedImports] Support string type annotations
This PR adds support for detecting imports being used by string type
annotations, as well as imports suppressed by comments.
It breaks up the existing visitor into multiple smaller, single-purpose
visitors, and composes them together.
Basically move the apply_annotations code from Pyre.
Make it a ContextAwareTransformer named ApplyTypeAnnotationsVisitor. Use GatherImportsVisitor to collect imports. Add a static method `add_stub_to_context` so that users can schedule a stub whose types are to be applied later. Use `assertCodemod` and `data_provider` in the tests. Add documentation. Remove fixmes.
