2021 compatibility adjustments

[MostHated]

Hey there,
Not sure if you are interested, but I cloned this and made a copy then made the adjustments necessary to work with 2021.5f1 by splitting things up with appropriate asmdefs to allow gaining access to the, now internal, CollectionHelper.CheckIsUnmanaged<T>(), updating the packages, etc.

If so, let me know and I can try to do a PR with it.

Thanks,
-MH

[TackJordy]

Hi how did you fix this to work with unity 2021.5f1?