Perform more transformations of parameters when generating Rust-side C functions

[Manishearth]

Currently we don't do any complicated munging of the signatures at the Rust level, the generated C functions almost always directly call the Rust functions without ceremony.

We should support some transformations, like:

• Returning Result<T, E> will generate a C API using DiplomatResult with the appropriate conversion at the boundary
• Self in parameters should be appropriately translated