Move AlreadyPrintedSet from STPManager to LispPrinter and eliminate IsAlreadyPrinted and MarkAlreadyPrinted from ASTNode.
Merge pull request #32 from khooyp/remove-already-printed

Move AlreadyPrintedSet from STPManager to LispPrinter and eliminate IsAlreadyPrinted and MarkAlreadyPrinted from ASTNode.