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

[tv42]

Can this ancient bug be closed? It seems using a build.rs build script and include! of files from OUT_DIR is the standard way to do code generation for Rust these days, and capnpc makes that as easy as it's anywhere.

(I do wish we didn't need the C++ binary, but that's not what this issue is about.)

[dwrensha]

This issue was opened in September of 2014. In November of 2014, capnproto-rust started using a build.rs build script, which I guess must have been a newly introduced feature of Cargo around then: capnproto/capnp-rpc-rust@33ac799

I could still imagine someone wanting to use a procedural macro, like capnp_mod!(example.capnp) to avoid needing an explicit build script, but it's unclear whether that's what this issue is exactly asking about. If someone wants that, they can open a new issue (but be advised that I prefer to avoid proc-macro).