samples: add Terra samples

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

samples/Terra/structsyntax.t

@@ -0,0 +1,14 @@
+struct A {
+    a : int
+    b : float
+    union {
+        c : int
+        d : float
+    }
+}
+
+terra foo()
+    return A { a = 4, b = 5, c = 3}
+end
+
+assert(foo().c == 3)