function::operator= should "move", copy assignment should have by-value argument

[niels_dekker]

A few days ago, I added a comment to ticket #1910 (regarding function::swap), suggesting to have boost::function's assignment operators calling its new move_assign member function, instead of calling swap. Doing so would significantly improve its performance.

Now I think that function::operator= deserves its own ticket, especially because the copy assignment of boost::function can be improved even more, by having its argument passed by value, instead of creating a copy of the argument inside the body of the function. Doing so would allow the compiler to do copy elision, when its argument is an rvalue. See also ​Improving the assignment operators of various Boost types

So please consider the attached patch.

[Antony Polukhin]

Your patch adds memory leaks for cases, when function is already initialized with big functional object (move_assign member function does not call clean()).