Add basic iterator/find support

[Jake-Shadle]

ZipArchive is a bit cumbersome to use when you are consuming archives with unknown/variable contents, so this PR exposes 3 helper methods on ZipArchive to make it slightly easier to consume such content.

• iter_files() - Returns an iterator over the internal vector of ZipFileData, which allows people to use all of the standard iterator goodies they are used to
• by_id() - As with the existing by_name/by_index, returns the ZipFile if that id exists. The id can be converted from a &ZipFileData and is unique (not name based)
• find() - While iter_files().map() + by_id() or iter_files().position() + by_index() can be used to find a file matching criteria, they are a bit cumbersome to use due to the borrow checker since iter_files() is an immutable borrow and all of by_ methods are mutable, so find just wraps the most common case of "get me the file that matches this predicate, if it exists"

[repi]

Any updates or ideas on this to make it acceptable and merged in?

[mvdnes]

I am sorry, but due to a lack of time I don't think I will be able to review anything from this library anytime soon.

[steven-joruk]

Sorry for going off topic Jake, I would like to see the iter part of this MR go in - @mvdnes would you consider adding other maintainers?