Remove the code-generation plumbing. (#1)

Now that Go has added support for parametric types, there is no longer any
practical reason to have the code generation plumbing here.

- Bump the Go toolchain base to 1.18.
- Remove the code generation script.
- Clean up generator artifacts in the code.
- Split the tests into a separate package.

This change does not modify the API of the core stringset package.