fixed names in Conversions
updated guava lib to ver 17
fixed name in DefinedFunctionsCreator
added NIL to PARAM_TYPES, it is allowed param types so it is possible to implement functions converting NIL value to other array types
redesigned StringExpr/StringWorker -> ConstantExpr/ConstantWorker
added Empty.typeFunctionMap()
added .lang.expr.Convert that provides converting expression implemented as NativeFunctions
switched from Converter interface to expr.Convert using NativeFunctions
removed obsolete Converter/Conversions