update tests to use DocField.apply
Nicer toStrings for physical plan (Fixes #125)
emit ADT constructors for selectors in RenderTree
Now that we have a separate mechanism to get the JavaScript form, this seems more useful
prefix ExprOp toStrings
prettify Sort toString
prefix Selector toStrings
Merge pull request #127 from mossprescott/wip/125

Nicer toStrings for physical plan