This is an attempt to make boost::atomic<> interface closer to the standard. It
makes a difference in C++17 as it mandates copy elision, which makes this code
possible:
boost::atomic<int> a = 10;
It also makes is_convertible<T, boost::atomic<T>> return true, which has
implications on the standard library components, such as std::pair.
This removes the workaround for gcc 4.7, which complains that
operator=(value_arg_type) is considered ambiguous with operator=(atomic const&)
in assignment expressions, even though conversion to atomic<> is less preferred
than conversion to value_arg_type. We try to work around the problem from the
operator= side.
Added a new compile test to check that the initializing constructor is implicit.
