samples: add Terra samples

source: https://github.com/zdevito/terra/tree/master/tests

samples/Terra/defaultoperator.t

@@ -0,0 +1,9 @@
+
+local a = "+"
+local b = "__sub"
+local ops =  {4,5}
+terra foobar()
+	return operator(b,operator(a,3,operator("+",ops)))
+end
+
+assert(foobar() == -12)