These conversions don't exist in std::shared_ptr-alike types.
But I don't want to bother with == operators until we have proper C++20.
Removed trivial conversion for atomic_ptr because it's heavyweight.
Fix atomic_ptr load() edge case.
Implement atomic_ptr::peek_op() to make possible to reduce load() overhead.
Implement atomic_ptr::compare_exchange() and friends.
Implement null_ptr constant, remove nullptr_t assignment/construction.