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_copy.cpp

@@ -15,7 +15,7 @@
 int
 main()
 {
-    test();
+    test<test_detail::can_copy>();
     return boost::report_errors();
 }