Macros have scope

After this patch, macros declared in a module, function, or block can only be used inside of that module, function or block, with the exception of modules declared with the #[macro_escape] attribute; these modules allow macros to escape, and can be used as a limited macro export mechanism.

This pull request also includes miscellaneous comments, lots of new test cases, a few renamings, and a few as-yet-unused data definitions for hygiene.