Make C++11 fusion::vector more consistent with C++03 fusion::vector

- Construct from elements is enabled iff each argument is_convertible to
    corresponding element.
  - Construct from sequence is enabled iff the single argument is a fusion
    sequence.
  - C++11 vector and tuple also disable construct from sequence that are
    shorter than the destination. C++03 gives incorrect is_convertible
    responses in this situation and fails to compile if that constructor is
    used; C++11 can have instantation errors in
    and_<is_convertible<U, T>...> without the additional check.
  - C++11 tuple and vector support truncation conversion and assignment like
    all other sequences.

test/sequence/vector_traits.cpp

@@ -0,0 +1,32 @@
+/*=============================================================================
+    Copyright (C) 2016 Lee Clagett
+
+    Distributed under the Boost Software License, Version 1.0. (See accompanying
+    file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+==============================================================================*/
+
+#include <boost/detail/lightweight_test.hpp>
+#include <boost/fusion/container/list.hpp>
+#include <boost/fusion/container/vector.hpp>
+
+#define FUSION_SEQUENCE boost::fusion::vector
+#define FUSION_ALT_SEQUENCE boost::fusion::list
+#include "traits.hpp"
+
+int main() {
+    test_convertible(true /* has conversion construction */ );
+
+    // C++11 models overly aggressive (bug) implicit conversion from C++03
+    BOOST_TEST((
+        is_convertible<
+            boost::fusion::list<int>
+          , boost::fusion::vector< boost::fusion::list<int> >
+        >(true)
+    ));
+
+#if defined(FUSION_TEST_HAS_CONSTRUCTIBLE)
+    test_constructible();
+#endif
+
+    return boost::report_errors();
+}