Are the map bindings in gen/let actually solving a problem?

I just discovered that the problem that the map binding form in gen/let was meant to solve is apparently not as big a problem as I thought, and possibly isn't a problem at all. It warrants more investigation before committing to that syntax in the final release.

The generator below shrinks just fine despite using bind, probably because the generators are independent and we pass the same immutable RNG in to the second phase when shrinking.