Support for aliases in using directives.

[bennor]

This fixes #61.

[anaisbetts]

@bennor How come this doesn't result in a change in RefitStubs.cs in the tests? Even though this is a generated file, I check it in so that we can see the history of the generated output (i.e. what was expected for that revision of the code)

[bennor]

I started out with a class that called RestService.For<INamespaceCollisionApi> (triggering the interface to be stubbed), but removed it because the code it was generating (before I added support for aliases) wouldn't compile, effectively breaking all of the tests.

I'm happy to add it back though, now support's been added.

[anaisbetts]

@bennor Please do then this should be good to merge ✨

[bennor]

All done 😸

[anaisbetts]

Thanks @bennor!