added helper methods to simplify test cases
Typechecker added (includes symbol table)
added testcases for typechecker
refactored AST nodes into different packages
renamed ASTNodeVisitor to Visitor
Added typeOf and isCompatible methods
made the Visitor generic
Implemented TypeChecker, refactored ASTPrinter and added testcases.
removed bugs from typechecker
refactored TypeChecker and TypeCheckerVisitor into one class
created typed ErrorList
Merge branch 'master' of https://github.com/software-engineering-amsterdam/sea-of-ql