Assist parent/child hydration

In practice it's quite common to want to select from a map key, or value. For example, when populating parent objects from child objects, you'll often have a map of `map[ParentID]bool` which you then have to flatten into a `[]ParentID` in order to select from.

Conversely, it's also common to need to be able to use the output of a select as a map in Go, eg. `map[ParentID]*Parent`.

It would be nice to have some clean way of doing both of these things.