Publishing to crates.io

Publishing compiler-builtins to crates.io takes a few steps unfortunately. It's not great, but it works for now. PRs to improve this process would be greatly appreciated!

1. Make sure you‘ve got a clean working tree and it’s updated with the latest changes on master
2. Edit Cargo.toml to bump the version number
3. Commit this change
4. Run git tag to create a tag for this version
5. Delete the libm/Cargo.toml file
6. Run cargo +nightly publish
7. Push the tag
8. Push the commit
9. Undo changes to Cargo.toml and the libm submodule