Introduce KClass.members, make properties/extensionProperties extensions

To avoid significant growth of KClass and KPackage interfaces

compiler/testData/codegen/boxWithJava/reflection/javaStaticField/K.kt

@@ -4,6 +4,8 @@ fun box(): String {
     val f = J::x
     assertEquals("x", f.name)
 
+    assertEquals(f, J::class.members.single { it.name == "x" })
+
     f.set("OK")
     assertEquals("OK", J.x)
     assertEquals("OK", f.getter())