Bag data structure proposal

Hello,

I noticed that you sometimes accept substantial pull requests to your code and that your library doesn't include a bag (an unordered, random insertion list). My bag is inspired by the C# ConcurrentBag, however my current implementation is not thread safe (should it be?). I have created a prototype that I could generalise to fit your API, so I wanted to check if you would consider a submission before I make the necessary changes.

Do you have any thoughts? Thank you and happy new year!