Add a syntax extension

[emberian]

Especially with cargo, generating code using tools can be annoying. Add a syntax extension that will generate the code into a module. With include_str etc, no flexibility is lost.

[dwrensha]

This would be really cool -- much better than my current hack for getting Cargo to do code generation, which you can see in these two repositories:

https://github.com/dwrensha/capnp-rpc-rust https://github.com/dwrensha/acronymy