Refactor operations

[JamesGallicchio]

Big refactor of Ops.lean.

• Improves class names to be consistent/predictable
• Refactors Fold correctness to be in terms of ToMultiset instead of list permutations. This includes some lemmas about multiset induction to prove properties of a fold.
• Adds ToSet class as the top of the collection model hierarchy

[JamesGallicchio]

Need to add deprecation notices to all the names that have been changed (eek)