backend: Use simple DFS during closure building.

Now closure builder can contain includes. Includes
are another builders which closures must be added
to the building one.
Includes are:

For function closure:
	call targets (including constructors)
	callable reference targets

For class:
	all children declarations
	call targets (including constructors)
        callable reference targets

For constructor:
	call and callable reference targets
	delegating constructor call target
	constructing class closure

backend.native/tests/codegen/function/localFunction.kt

@@ -7,5 +7,17 @@ fun main(args : Array<String>) {
         }
         local0()
     }
+
+    fun l1() {
+        var x = 1
+        fun l2() {
+            fun l3() {
+                l1()
+                x = 5
+            }
+            l3()
+        }
+    }
+
     println("OK")
 }