samples: add Terra samples

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

samples/Terra/usercast.t

@@ -0,0 +1,24 @@
+
+struct A { a : int, b : float }
+
+function A.metamethods.__cast(from,to,exp)
+    if from == int and to == A then
+        return `A {exp, 1.f }
+    elseif from == float and to == A then
+        return `A { 1, exp }
+    end
+    error("invalid")
+end
+
+
+terra moo(a : A)
+    return a.a + a.b
+end
+
+terra bar()
+    return moo(1) + moo(1.f)
+end
+
+local test = require("test")
+
+test.eq(bar(), 4)