impl IntoIterator for {&,&mut}Arena

[toyboot4e]

Currently, Arena::iter or Arena::iter_mut have to be called to use iterators. This PR adds trait impls that allow idiomatic loop syntax: for .. in &arena.

Example:

use thunderdome::Arena;

fn main() {
    let mut arena = Arena::new();
    arena.insert(1 as usize);
    arena.insert(2);
    arena.insert(3);
    for (index, item) in &arena {
        println!("{:?}, {:?}", index, item);
    }
}

Output:

Index { slot: 0, generation: Generation(1) }, 1
Index { slot: 1, generation: Generation(1) }, 2
Index { slot: 2, generation: Generation(1) }, 3

The iterators return (index, item) pairs, but it's not common, so item-only iterator can be preferable.

What would you think? Thank you.

[LPGhatguy]

These iterators look perfect to me! Can you also add an entry to CHANGELOG.md to mention that these iterators were introduced?

[toyboot4e]

Yes, I commited now :)

Edit: Feel free to modify my PR. I'm not sure if I did it right, actually.

[LPGhatguy]

Thank you!