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.