Add failing tests for string operations.
Add support for string operations.

`substring`, `lower`, and `upper`. Also added `length`, but it has no implementation on the Mongo side.
Add falling test for modulus.
Implement modulo.
Add failing tests for coalesce.
Implement coalesce/$ifNull.
Fix Cond with non-Const condition.
Merge pull request #117 from sellout/63-additional-operators

Add support for additional operations.
expand nested Reshapes
assemble sort keys into an array, and then sort on its elements, each on the right order
tests for filter with expr in projection (pending);
update and enable test for sort with wildcard and expression in sort key
don't show the stack trace in trees
make failure messages useful
RenderTree.diff()
use String instead of Cord in RenderedTree (because it implements equal(), and these labels should never be large anyway)
RenderTree instance for Term
update tests
use MakeArrayN consistently
update sort/filter expectations
show RenderTree diffs in planner test failures
tests for RenderTree.diff