Changed some annotations to @Nullable to make them JDK-compliant
Index object declarations as inheritors
Add local class/object declarations to super name index
Add support of light classes for local class/object declarations
Translate KotlinDirectInheritorsSearcher to Kotlin. Reuse PsiClass.isInheritor()
Move search extensions to separate package
Core classes for declaration search
Implement class inheritors search
Reimplement "Find Usages" using KotlinClassInheritorsSearch
Rename "Find Usages" tests
Add "Find Usages" tests for local classes/objects
Fix warnings