But code written using this library is no longer Go: most Go programmers can't grok it, and it's awkward to call normal Go libraries because there's no way to know if that function you're calling is pure.
If your goal is to "make it easy and fun to write maintainable and testable code in golang" by making pure functions first-class, is there another way to do that without inventing a new language?
From experience and inspired by Carmack's classic essay on FP in C++[1], I tend toward a functional style: minimize state, treat locals as const, avoid non-const globals, enable parallelism by isolating state. Go makes it easy to write static analysis tools, so go vet could be augmented to, for example, keep track of which functions are pure, and show some yellow underlines at those places where input parameters are mutated.
I'd use something like that.