A few random fixes.

* give `ManyErrors` a useful stack,
* implement `IN` and `NOT IN` (two new integration tests), and
* fix the `NumericUnary` untyper (new compiler test and pending (#351)
  planner test).
Merge pull request #352 from sellout/ready/minor-issues

A few random fixes.
pull out workflowop merge as ++ operator
experiment with test of WorkflowOp-level merging
in progress