Package fix/takeover: AC-Vector

Hello, Here's my attempt to take over an apparently unmaintained package: https://hackage.haskell.org/package/AC-Vector It has been suggested to me to do this here: https://github.com/haskell-infra/hackage-trustees/issues/391 I have prepared a patch rendering the instance declarations in code compatible with the class structure of base>=4.9. If any authorised person were to simply apply the enclosed patch, I'd appreciate that too. That's all, Jan