Map "clear" method.

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