Move semantics for Nat, Interval and Division

[saxbophone]

Create move constructor and move assignment operator for Nat. I think we will not need to create them manually for Nat itself, nor for Interval or Division because all of these satisfy the requirements for generating implicit move constructors for them. However, to support this, we need to make sure the type used to store digits does have a move constructor/assignment operator.

Prerequisites:

• #136
• #137
• #133
• #131