Add Arena::map

[Ratysz]

Analogous to Iterator::map(), array ::map(), etc...

My usecase is pretty much typestate pattern; I want to shed parts of stored items that aren't useful anymore, but I also want existing indices to remain valid.

Alternative approach is to allow constructing Arena<T> from iterator of (Index, T). This could also help with implementing serialization further down the line.

[LPGhatguy]

We talked about this with @sdleffler on Discord a little bit. I think this functionality is useful, but I'd like to explore the option of being able to collect from an iterator of (Index, T) before we decide either way. If that approach has no obvious downsides, it seems preferable to me.

[Ratysz]

Sounds fine by me; in fact, I would prefer that as well, as it's strictly more versatile. Don't hesitate to close this whenever, should you settle on that approach.

[LPGhatguy]

Hi, I wanted to post an updated comment on this.

Years later, I still think we should not have Arena::map (because other allocating data structures like Vec don't) but we should definitely implement FromIterator like I alluded to years ago. I would accept a PR from anyone interested to do that!