- base class method wins against a (default) interface method,
so an abstract base class method should always be implemented
in a derived class;

- interface methods clash regardless of abstract/default
with possibly undefined behavior at run-time,
so a class or interface should always define its own method
for methods inherited from multiple interfaces and not from base class;

- meaningful diagnostics for class inheriting conflicting JVM signatures.
Since no override will happen under Java 8 rules,
ACCIDENTAL_OVERRIDE is misleading for this case;

- update testData.

 to CandidateResolver

 Added error SUPER_CANT_BE_EXTENSION_RECEIVER (it was SUPER_IS_NOT_AN_EXPRESSION)

- test data files renamed from *.jet to *.kt

otherwise I have to rollback dozens of files after using sed that follows conventions