KT-6120 Java to Kotlin converter can generate mutable collection types in case if collection write methods used

 #KT-6120 Fixed
Java to Kotlin converter tests: added check for errors in result file
J2K converter testing: added dumping of errors into multifile tests too
J2K: renamed Field to Property
J2K converter: automatic generation of 'open' modifier for classes and methods
J2K: fixed assertion
Minor
J2K: correct mutable/non-mutable types for overrides of Kotlin methods
Renamed LazyElement -> DeferredElement
J2K: singletons are back
J2K: fixed usage processing scheme to cover exotic case with anonymous class
J2K: replaced IntermediateResult with function
Java to Kotlin action: no infinite recursion for symlinks + more efficient update()