Fixes re: Moss’ feedback.

* remove `toString` from rest of ExprOp (except `DocVar`),
* add result type to `side` function,
* replace a `flatMap` with a `join`,
* rename `meh` to `convert`,
* put `GroupContents` cases into the companion object, and
* improve `WorkflowBuilderRenderTree` (but doesn’t yet expand `ListMap`s).