Added Expression Type Checker Unit Test
Removed accidental infinite loop from Literal Expr type checker visitor
Added remaining Unit Test cases for arithmetical type checking
Added Helper method for type checking in Unit Test for better readability
Refactored Unit Test to use assertTrue/False, invalid typecheck helper method
Moved helper method implementation details to generic lower level helper method
Created super class for Expression Type Checker
Refactored binary numeric expr type checker, now extends super class
Moved expression parsing implementation details to super class
Refactored Expr type checker to parse through super class
Refactored super class so only string input is needed for expr type checking