samples: add Terra samples

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

samples/Terra/unm.t

@@ -0,0 +1,20 @@
+
+
+struct A {
+	a : int
+}
+
+A.metamethods.__unm = terra(self : &A)
+	return A { -self.a }
+end
+
+A.metamethods.__sub = terra(self : &A, rhs  : &A)
+	return A { self.a - rhs.a }
+end
+
+terra doit()
+	var a,b = A { 1 } ,  A { 2 }
+	return (-(a - b)).a
+end
+
+assert(doit() == 1)