The library is Go. What you're suggesting seems to be extending Go so that it becomes some other language derived from Go. Extending Go might make for a cleaner and/or better generics solution but if you have to have customized compiler to run the code then you're no longer running Go.
Historical tidbit: Pizza is the precursor to Java's own generics implementation (introduced in Java 5), both were written by Martin Odersky who then went on to create Scala.
