Implement Rust initialization macro

[camelid]

Rust proc macros must be defined in a crate that exports only proc macros, so
there are now two crates. I re-exported the macro from the main crate, so users
will never need to explicitly depend on it. I also set the second crate's
version to be the same as the main one since they should be kept in sync.

[camelid]

Rustfmt automatically made some formatting changes. I can put them in a separate commit or revert them if that would be helpful.

[nicovank]

LGTM, thanks a lot, this is nicer!

[camelid]

Thanks for reviewing!