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:

• Interval<T> type for return results of ilog, ipow, iroot #136
• Division<T> type for representing result of division with remainder #137
• Store digits using more efficient container #133
• Check if std::move needs to be used to move digits in some methods #131