[boost] function_equal and function::contains committed

Based on the discussion with Peter Dimov and Brian McNamara, I've checked in boost/function_equal.hpp with the new function_equal function. Overload boost::detail::function_equal_impl with a final argument of "int" in libraries that need to be compatible with older compilers. function_equal is documented as part of Boost.Function (because it needed a home somewhere inside BoostBook). function<> now has a contains() member function and uses function_equal instead of operator ==. Doug