Add move semantics

[viboes]

Boost.Tuple lack move semantics for its arguments. It should be great if it can comply with the c++11 standard when the compiler provides rvalue references and use Boost.Move otherwise.

[awulkiew]

I've created experimental version of tuple supporting move-semantics here:

​http://svn.boost.org/svn/boost/sandbox/tuple-move/

To ensure backward compatibility I've defined copy assignment explicitly. It's possible to use tuples wrapped in classes with implicit assignment operator in c++03:

struct wrapper
{
  tuple<int> t;
};
/*...*/
wrapper w;
w = wrapper();

however in c++03 move assignment won't be used if tuple is returned from a function by value.

tuple<int> ret_tup(int a) { return tuple<int>(a); }
/*...*/
tuple<int> t;
t = ret_tup(0); // copy in c++03

We could add a define check that disables mentioned explicit copy assignments. Defining it would enable moving of objects returned by value but prevent compilation of wrappers with implicit copy assignment.