Cross-crate exporting and reuse of templates (and drivers)

This also includes:

• tests/pub-export: a test/demo of cross-crate reuse of a template and a driver, including:
• Test that each crate gets the template syntax for its derive-adhoc dependency, using
• A "bizarre" scheme for providing a mutated template language from the same source code
• Some future-proofing in the internal macro arguments
• A new ${crate} expansion keyword for the use of an exported template

What is missing is public-facing documentation. There are some TODO DOCS comments. Hopefully the commentary in the tests/modules.rs and tests/pub-export source files will be helpful in the meantime.

I won't merge this now since I don't want to make main diverge so much from the recently released and very-recently-announced 0.1.0.