feat: implement map clear method to remove all entries

Add DEF_NATIVE(map_clear) that frees the underlying entries array and resets
capacity and count to zero, returning null. Register the "clear" method on
the Map class in wrenInitializeCore and include a test verifying the map is
emptied and the return value is null.

test/map/clear.wren

@@ -0,0 +1,7 @@
+var a = {1: 1, 2: 2, 3: 3}
+a.clear
+IO.print(a)       // expect: {}
+IO.print(a.count) // expect: 0
+
+// Returns null.
+IO.print({1: 2}.clear) // expect: null