samples: add Terra samples

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

samples/Terra/toterraexpression.t

@@ -0,0 +1,8 @@
+local x = setmetatable({a = 4}, { __toterraexpression = function(self) return self.a  end})
+
+
+local terra foo()
+    return x
+end
+
+assert(foo() == 4)