List.clear and List.insert().

test/list/clear.wren

@@ -0,0 +1,7 @@
+var a = [1, 2, 3]
+a.clear
+io.write(a)       // expect: []
+io.write(a.count) // expect: 0
+
+// Returns null.
+io.write([1, 2].clear) // expect: null