Rewrote enum lowering to support usage from other modules.

- Split up enum lowering into two stages: descriptors creation and IR creation.
- Added link stage test.
- Enum.valueOf() uses binary search instead of linear.

backend.native/tests/codegen/enum/linkTest_main.kt

@@ -0,0 +1,7 @@
+import a.*
+
+fun main(args: Array<String>) {
+    println(A.Z1.x)
+    println(A.valueOf("Z2").x)
+    println(A.values()[2].x)
+}