make rustdoc handle imports and exports

[boggle]

rustdoc should copy docs from imported symbols. this would, for example, help float get docs from f32/f64.

Probably this should wait/ties in with copying default docs from interfaces when typeclasses become available.

[brson]

This is going to require some heavy refactoring of the resolve pass. Right now the only way to get at this information is to run all of resolve, which will fail if all the dependencies aren't built and locatable. Requiring a build just to extract docs seems like a bad situation.

I also opened another issue for copying interface docs to implementations (#1678) because that will be harder still.

[brson]

Done.