Support imports from other crates

[shadaj]

When we add support for wildcards, this can be handled similarly by having a fallback that attempts to look up the first symbol in a path by resolving it to a crate declared in some Diplomat config.

[Manishearth]

I don't think we need to have support for wildcards for Diplomat imports?

Non-diplomat imports, definitely.

[sffc]

We can start throwing errors after we add annotated imports (#31). Cross-crate imports could be a future enhancement.

[Manishearth]

The way this would probably work is that diplomat-tool can dump json specs for various crates, and these specs can be fed back to diplomat-tool which will add the json specs to its import graph.

We should not be trying to automate this further: trying to magically run diplomat on the entire dependency tree will have the same problems cbindgen does. For a given application, whoever is running diplomat-tool will already know where the diplomat-using crates are and they can easily sequence it.