Updates the grammar. Return statements can now only be in function bodies.
Moves around the tests to better match the order that they appear in the grammar file. Also updates the tests to check for variable indices rather than a constant 0.
Moves around the visitor methods to better match the order that they appear in the grammar file.
Adds the implementation of VisitNormalIdentifierExpression and the tests for it.
Adds the implementation of VisitPropertyIdentifierExpression and the tests for it.