Assume a value for AUTOCXX_RS

[adetaylor]

Following #171 our macro now simply include!s a file generated in an earlier codegen phase.

It requires AUTOCXX_RS to be set to educate it where to find such files, but in a cargo context it should be able to assume a default.

It seems quite likely that rust-analyzer and friends will be able to parse and understand the FFI if we manage to do htis.

[adetaylor]

Fixed, but didn't perfectly do the trick. rust-analyzer can now find the generated .rs file and that's terrific; but it doesn't attempt to expand the #[cxx::bridge] macro inside it. Ideally we'd take this forward by providing a minimal repro to rust-analyzer, and perhaps I'll do that in future.