make Arena::next_index public.

[sanbox-irl]

This makes Arena::next_index a public function, which enables knowing the next index which will be used by the arena without owning exclusive access to the arena.

Note this cannot be merged until #57 is merged.

[LPGhatguy]

This is where that cool new "stacked PRs" feature would be nice, huh.

[sanbox-irl]

@LPGhatguy yup. rebased for now.

I should say that I tried to make an insert_with_key function here too, but it was just going to wrap the implementation of Arena::vacant_entry, and I realized it wasn't worth doing.

This method gives the user something new -- namely the ability to get the next key immutably. I suspect some users won't need that and getting only mutable access will be just fine, but I doubt it!