Implement CROSS.

Blows up memory like nobody’s business, but the semantics should be right.

And move `WorkflowBuilder.join` to the class, like other similar functions.

Also adds a test and rejiggers the `joinStructure` helper function yet again.
Merge remote-tracking branch 'upstream/master' into cross
Merge remote-tracking branch 'upstream/master' into cross

Conflicts:
	src/main/scala/slamdata/engine/physical/mongodb/workflowbuilder.scala
&&& snuck back in during the git merge.
Merge pull request #270 from sellout/cross

Implement CROSS.