`ygm::container` variable reassignment using `std::swap`

[bwpriest]

It would be valuable to reassign container objects inline using the = operator. For example, I have workflows where I iteratively build containers while discarding old versions. It would be very convenient to do so via something like:

ygm::container::map<key_type, value_type> my_map = make_map();
while(condition == true) {
  my_map = make_new_map(my_map);
}

This does not currently work, and I believe we need to add an overloaded = operator for each container with an implementation like

container_type &operator=(container_type rhs) {
  std::swap(*this, rhs);
  return *this;
}

We will probably need to do some accounting with the ygm_ptrs for the objects in this case, though.