Now that this package has a minimum toolchain that supports the package access modifier, we are no longer forced to use code generation to avoid making internal helpers shared across modules public.

De-gyb all affected source files and remove the entire gyb infrastructure from this package. 🎉

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.