Add // for Data.Sequence

[Alan](http://stackoverflow.com/users/2913864/alan) has suggested [on StackOverflow](http://stackoverflow.com/questions/37505725/standard-queue-package-for-haskell/37506254?noredirect=1#comment62521038_37506254) that we add a `//` operator to `Data.Sequence` corresponding to the one in `Data.Array`. I believe that doing this efficiently likely requires access to `Data.IntMap`. We don't have that in `Data.Sequence` right now, but I'm hoping maybe the restructuring expected for the next major version after the upcoming one will allow us to do this.
### Implementation sketch

Collect all requested changes into an `IntMap`, then use something similar to `Data.Sequence.splitMap` with `Data.IntMap.split` to spread them through the tree. `splitMap` itself is overkill, because we'll likely be able to preserve whole subtrees. We may need to add some more splitting functions to `Data.IntMap`, if it supports them, but we probably want to do so anyway to match up with `Data.Map`.