Introduction of sealed classes

Sealed classes can be derived only by their own inner classes or objects.
Their constructors cannot be called explicitly, so compiler knows all their descendants.
Incompatible modifier checks (final, abstract). Impossible with interface, object, enum.
A pack of tests provided.

compiler/testData/diagnostics/tests/sealed/NeverObject.kt

@@ -0,0 +1,3 @@
+<!ILLEGAL_MODIFIER!>sealed<!> object Sealed {
+
+}