Decide what to do around `mod ...;`

[harpocrates]

Rust supports two syntaxes for mod - one for declaring inline a module and another for referencing the contents of another file. We don't currently try to go follow those files (since that requires IO and parsing is otherwise pure).

• Add an IO based parse that chases down mod ...; items to expand them
• Think about what to do in tests (right now, I have to manually take out mod ...; forms to make the difference tests pass