Add Transaction#mutated?

[no-reply]

This would check whether the Transaction has any changes from the target Repository.

For the implementations we have now, the check would be easy to implement and cheap:

• For the base class, it's changes.inserts.empty? && changes.deletes.empty?;
• For the SerializableTransaction it's a simple object equality check on two Hamster::Hash instances.

For other transactions, things might not be so easy. We could recommend NotImplementedError in that case, or provide an API like #<=>'s nil for "not comparable".

As a motivating use case, see: samvera/ldp#28 (comment)

[artob]

I would generally prefer #mutated? over #changed?, as a more specific term. ("What changed? Oh, it was mutated.")