Enumeration iterators for the Repo collection

[Eliah-Lakhin]

Resolves #29

Introduces three utility function to the arena::Repo collection:

1. Repo::enumerate - yields pairs of (Entry, &T) for non-exclusive iteration.
2. Repo::enumerate_mut - yields pairs of (Entry, &mut T) for exclusive iteration.
3. Repo::into_enumeration - yields owning pairs of (Entry, T).