Add Stmt::LowestCoveringSubstitution() method, binary, and tests

[PappasBrent]

• Add the method LowestCoveringSubstitution() to pasta::Stmt for getting the lowest expansion that completely covers a given Stmt.
• Add a binary, print-lowest-covering-substitution, for testing this new method.
• Add tests for this new method.
• Add a few helper methods to the MacroRange and MacroSubstitution classes.
• Refactor some existing code to make development a little easier. See commit messages for detailed explanations.