Scan the entire crate for exported macros

[sfackler]

It previously missed anything in an inner module.

[alexcrichton]

Hm, is this the right thing to do? I thought that the macro_export macro simply lifted the macro's scope up one module. So in the test case you added it lifted the macro up to the crate, but it didn't lift it beyond the crate (the crate itself doesn't have the macro_export attribute).

@jbclements would probably know more about it, but I figured that this exportation would take macro scope into account?

[sfackler]

@alexcrichton macro_export exports cross-crate. macro_escape lifts the macro up one module.

[alexcrichton]

Whoops, my mistake!