Typed bounds for iteration

[uint]

The start and end parameters currently take byte strings. These should be keys.

For now, let's take the "partial" rather than "full" key. It's a much easier-to-use API and likely to be enough for most use cases. It shouldn't be difficult to redesign in the future if proven otherwise.

let my_map = Map<String, Map<String, Item<u32>>>;

let keys = my_map
    .access(&storage)
    .keys(Some("foo"), Some("bar"));                                   // partial
let keys = my_map
    .access(&storage)
    .keys(Some(("foo", ("xyzzy", ()))), Some(("bar", ("zzyxy", ())))); // full

Probably more a "good to have" than a "must" for the prototype.