Add a Data.Sequence.Strict

`Data.Sequence` is only strict in its length and not its values - analogous to how `Data.Map.Lazy` is strict in its keys but not its values.

This is causing a space leak in my program, and ghc-heap-view clearly shows unevaluated thunks within the `Seq` structure. The solution that is most friendly to users of `containers` such as myself, would be to provide a `Data.Sequence.Strict` that forces the values before storing them inside the container.