Added a comment to build that shows how to get info about implicits to print out.
Refactored the InnerConstraints into more priority layers. I was trying to
solve a problem this didn't solve, but I decided it shoudl be this way anyway,
because we need to emulate exactly the priorities of regular Constraints,
for which the two TypeChecked ones will be found first since they are in scope.
Also added tests and code to handle Nothing as one of the types for non-nested Ors.