Integrated functionality of literal expressions into values
Refactored Value composite super class, now also extends Expression
Moved Identifier to expression package, integrated hashCode/equals
Fixed Evaluator visitor
Changed namespaces in Grammer to reflect changes
NullValue overrides typeOf and accept method of Value/Expression
Added NullValue visit method to typechecker interface
Changes in TypeChecker to reflect changes
Added new ExpressionValueChecker super class for unit testing values
Added unit tests for arithmetic expression value checking