Renames a test and adds a new test for a more complex expression.
Adds another test (currently failing) for addition including variables.
Updates the grammar a bit more (UnaryExpression and removal of Sign).
Adds an instruction for logical negation.
Adds implementation for the current unary operators and adds tests for VisitUnaryExpression. Also cleans up a lot of worthless tests (they will be in a more specific set of tests for arithmetic).