STLExtras.h: introduce IsTupleLike concept

One day we'll need to make it report error messages in a more effective
way.

tests/unit/Model.cpp

@@ -180,6 +180,8 @@ public:
 
 INTROSPECTION_NS(TestTupleTree, Root, Elements)
 
+static_assert(IsTupleLike<TestTupleTree::Root>);
+
 BOOST_AUTO_TEST_CASE(TestTupleTreeReference) {
   using namespace TestTupleTree;