Convert JsExprPhase to “Arity” style.

Fixes some non-comprehensive pattern matches.
Fix PureOp merging.

This also allows us to toss the wart that is DummyOp.
Add a pending test for PureTask plan.
Merge pull request #269 from sellout/minor-fixes

Minor fixes
Merge remote-tracking branch 'upstream/master' into cross

Conflicts:
	src/main/scala/slamdata/engine/physical/mongodb/workflowbuilder.scala