samples: add Terra samples

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

samples/Terra/mixed.t

@@ -0,0 +1,13 @@
+a = terra() return 4 end
+local d = {}
+do 
+    struct A{}
+    and local terra a() return 5 end
+    and terra d.d(a : B) end
+    and terra B:a() end
+    and struct B {}
+    and terra c() end
+    and struct C {}
+    assert(a() == 5)
+end
+assert(a() == 4)