(frontend) Refactoring: StatementCall#parameters -> arguments
(TypeValidator) Remove useless argument when using Xtext error()
(refactoring) PrintlnTransformer uses correctly net.sf.orcc.util.Void
(validation) Binary operations with lists produce errors
(frontend) Don't use TypeUtil.isConvertibleTo(null, <something>)
(frontend) StmtTransformer doesn't return anything
(frontend) Refactoring: node(.*) -> block$1
Merge branch 'frontend_updates'

These commits perform some refactoring and fix minor issues.
It will be useful for the next changes in the frontend