Make union/except/intersect chainable as SelectManager

Fixes #118.

This is the same as #239, but against master instead of 4-0-stable. It would be really nice to get this in so we don't need to keep freshening it! Or if there's a reason it isn't being merged, feedback would be great.
I'm not sure (and tests pass with a shallow clone), but it is consistent with the definition in the `Binary` node, which clones each of its children.