Implement iterators for Map and Set

[cuviper]

This adds:

• Map::{iter, iter_mut, into_iter}
• Map::{keys, values, values_mut}
• Set::{difference, symmetric_difference, intersection, union}

We already have Set::{iter, into_iter}, and sets don't usually have iter_mut.

Fixes #1.

[cuviper]

For full parity with std:

• BTreeMap::{range, range_mut} and BTreeSet::range could be supported
• HashMap::drain and HashSet::drain might work -- even though kudzu doesn't support removal, it may be possible with exclusive &mut access.
  • same for retain, though that's not really an iterator, rather an iterating callback