AoC fixes and additions

[udoprog]

• Introduces ::std::collections w/ HashSet and HashMap
  collections.

These differ from what the anonymous object is capable of by using a
Key type similar to ConstValue.

This type gatekeeps values which cannot be used as a key in a hashed
collection, allowing for these collections to be used in situations
where complex keys are needed and not just strings.

use std::collections::HashMap;

enum Tile {
    Wall,
}

let m = HashMap::new();

m.insert((0, 1), Tile::Wall);

assert_eq!(m.get((0, 1)), Some(Tile::Wall));
assert_eq!(m.get((0, 2)), None);

• Introduces a couple of utility functions (these will hopefully use
  generics in the future to avoid the type in the name):
  • String::is_empty.
  • String::split (char pattern monomorphization).
  • ::std::string::parse_int.
  • Vec::sort_int.