Try to improve the fragility of attach order for intersect, union, setdiff w.r.t. {nanotime} #319

In 4.8.0, we exported intersect(), union(), and setdiff(), which as we saw, broke {nanotime} by changing how intersect() resolves depending on attach order.

We currently have a workaround for that to pass {nanotime} tests: eddelbuettel/nanotime#155

But the user experience remains less-than-ideal -- fragile/brittle, hard-to-diagnose for non-expert eyes.

We workshopped two approaches before release, neither of which were found to be satisfactory:

• Try to dispatch S4 if appropriate from bit64::intersect(): Attempt S4 dispatch for intersect() and union() #308. Ran into an undiagnosed segfault.
• Make bit64::intersect() S4 generic: Attempt S4 dispatch for intersect() and union(), maybe - alternative #307. I'd like to avoid introducing S4 just for this.

Let's see if something can be improved, including by potentially patching {nanotime} with a low-touch accommodation as well.

cc @hcirellu @eddelbuettel