Standard library should have a set module

[catamorphism]

There's code in rustc that represents sets as hash maps with value type (), and that's a little hokey. Sets should be in the standard library.

[Dretch]

It would also be nice to be able to pass in the hash/eq fns using ifaces, like:

// somewhere in core
iface eq { fn eq (a: self) -> bool; }
iface hash { fn hash () -> uint; }

// a set module constructor
fn mk_set <V:eq hash copy> () -> set<V> { ... }

// to construct a set of some type with eq and hash in scope:
let s = set::mk_set();
s.insert(...);

Actually the same thing would be nice for maps, too.

[catamorphism]

@Dretch That would be good, yes.

[catamorphism]

b86c90b is a start towards this, but the usual set operations still need to be added.

[catamorphism]

b927e48 closes this. Thanks, @thestinger !