Rust: add option for path to wit-bindgen runtime.

[peterhuene]

This PR adds an optional path to the wit-bindgen runtime to use for Rust code generation.

This will allow a different crate from wit-bindgen to generate bindings while also re-exporting wit-bindgen::rt at a known path for the generated code to use.

[pchickey]

Thanks! can you add a test which invokes this with e.g. pub mod my_rt { pub use wit_bindgen::rt::*;}?

[peterhuene]

I've added a runtime_path option to the generate! macro so we can set it via a test, which I've also added.

[peterhuene]

@alexcrichton thoughts on doing a release that might include this change?

This will enable cargo-component to migrate to using a wit-bindgen::generate!-style macro ahead of the resource changes and we've also rev'd the wasm-tools dependencies in main, which would be nice to have for downstream crates to sync on.

I realize that we'll want a release in the (near?) future to support resources, but an interim release would definitely help me out.

[alexcrichton]

Sounds good to me! I'll send a version bump to this repo which will be good to pull in the wasm-tools update regardless.

[alexcrichton]

Posted a bump at #620