Update for new enum scoping rules (#27)

The items in enumerations are no longer added to the module scope by
default.  We fix the code to work with this by adding 'use self::...'
declarations to export the enum members as needed.  For more
information, see:

https://github.com/rust-lang/rust/pull/18973

This also required adding a new 'use' statement to the generated
code, and updating grammar.rs accordingly.
Merge pull request #28 from emk/enum_scoping

Update for new enum scoping rules (Fixes #27)