Rewrite assignment operators using newer idioms

[htiek]

Our current assignment operator implementations use this general idiom:

 Object& operator =(const Object& rhs) {
     if (this != &other) {
         clear();
         deepCopy(other);
     }
     return *this;
  }

This has a few drawbacks:

1. It's not exception-safe. If something bad happens during the copy step, the internal state is corrupted.
2. It requires the logic for performing a deep copy to be factored out of the copy constructor.
3. It doesn't play well with move semantics.

The more modern C++ approach is to write an assignment operator like this:

Object& operator =(Object rhs) {
    swap(*this, rhs);
    return *this;
 }

This "construct-and-swap" idiom just requires the definition of a swap function that does a memberwise swap and lets the compiler decide whether to construct the argument using copy semantics or move semantics. There's no need to factor the copy logic out of the constructor.