added .task package with Task and friends
switched from using Expression to Task
removed obsolete Expression and friends
removed unneeded test
reused methods from Empty class in PrecalculatedTask
declaration of function name implemented by plugin moved from @FunctionName to @ExecuteMethod, removed @FunctionName
added TaskExecutorTest
redesigned Problem class hierarchy, removed SourceLocation from Problem, added CodeProblem, ErrorProblem, WarningProblem subclasses