Add Set Algebra iterators to trait Set: Intersection, Difference, Sym. Diff, Union.

[bluss]

These methods exist (or used to exist) in the Set trait, and should be reintroduced when we can express methods returning iterators in a trait.

/* These are the old methods */
/// Visit the values representing the difference
fn difference(&self, other: &Self, f: &fn(&T) -> bool) -> bool;

/// Visit the values representing the symmetric difference
fn symmetric_difference(&self, other: &Self, f: &fn(&T) -> bool) -> bool;

/// Visit the values representing the intersection
fn intersection(&self, other: &Self, f: &fn(&T) -> bool) -> bool;

/// Visit the values representing the union
fn union(&self, other: &Self, f: &fn(&T) -> bool) -> bool;

Bonus points for: Allow operations across two sets with matching element types but generic Set impl.

[pnkfelix]

Just to keep bugs appropriately linked: one hypothetical solution for expressing trait methods that return iterators is to make the iterator's type an associated type of the trait. A la #5033.

[steveklabnik]

We no longer have a Set trait, and so I'm giving this a close.