Don’t expose the dangerous WorkflowBuilder.appendOp.

(Previously known as `>>>`)
Implement math operations for JsExprPhase.
Implement filter properly.

Selectors now include a list of paths through history to find
WorkflowBuilders that are needed for input.

* Uses `$where` less frequently, but as a result can cause more
  aggregation steps and requests (would be nice to coalesce this into
  `$where` later, but could be difficult);
* eliminates the FieldPhase; and
* improves merging of ExprBuilders (which simplifies one existing test)
  and GroupBuilders without Object content.

This may also result in merging SelectorPhase and WorkflowPhase down the
line.