Support multiple cxx::bridge modules in one file

[dtolnay]

Following #370 this should be pretty easy hopefully.

• As far as the C++ code generator is concerned, I think multiple cxx::bridge is exactly equivalent to seeing all of the contents together in a single module, with appropriate namespace attributes if the two cxx::bridge invocations are different namespaces.

• As far as the Rust code generator (proc macro) is concerned, I think multiple cxx::bridge can expand totally independently (this probably already does the right thing).