We can just take the value parameter by value which allows both moving
into it, and copies at the same time, depending on the calling code.

These operators don't modify internal class state, so they can be made
const member functions. While we're at it, drop the unnecessary inline
keywords. Member functions that are defined in the class declaration are
already inline by default.

This provides the equivalent behavior, but without as much boilerplate.
While we're at it, explicitly default the move constructor, since we
have a move-assignment operator defined.