This introduces some fancy macro action that allows us to skip the four calls to `(concat-spec …)` with the different generators and types.
@ErikRauer and I wrestled with this for a _long_ time today, and it turned out that it was absolutely necessary to quote the generators in `gen-type-pairs on lines 10-13. I'm not 100% sure why, but it seems that without that something (perhaps a level of macro-ness) got unwrapped too early and then it couldn't find the generator because it had lost the namespace it belonged in.
Whether this actually _simplifies_ things is really up for debate. If this is as far as we get, it's probably not worth it. If we can find a reasonable way to reduce the other sources of duplication, however, it might be worth it?
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