Publishing rust-gpu on crates.io

This is a task list for the maintainers of rust-gpu to remember to do when publishing a new version of rust-gpu (probably not useful for contributors without access to embark's crates.io account 😋)

The published crates and their relative locations are:

1. spirv-std-types (crates/spirv-std/shared)
2. spirv-std-macros (crates/spirv-std/macros)
3. spirv-std (crates/spirv-std)
4. rustc_codegen_spirv-types (crates/rustc_codegen_spirv-types)
5. rustc_codegen_spirv (crates/rustc_codegen_spirv)
6. spirv-builder (crates/spirv-builder)

Publishing the crates in above order prevents dependency issues. These are the steps:

1. Bump all the versions to the next one in the workspace's Cargo.toml. This project uses workspace inheritance, so this is the only place you'll find these actual versions. Make sure to pin the rust-gpu dependencies to their exact versions using the =-notation, such as: =0.4.0. All crates are built and published in tandem so you're not expected to be able to mix and match between versions.
2. Add this new version to the table in crates/spirv-builder/README.md and make sure the correct nightly version is listed there as well.
3. Create a PR with that change. Wait for CI and a review, and merge it.
4. Pull the merged main branch.
5. Tag main with the version: git tag v0.4.0
6. Push the tag: git push origin v0.4.0
7. Publish the crates: cd [crate] && cargo publish in the order of the list above (make sure .cargo/credentials is set to embark's token). The crates.io index might take some seconds to update causing an error if the crates are published in quick succession. Wait a couple of seconds and try again 🙂.