Implement `GetSlice`

Like
``` F#
[<Extension>]
type SliceExtensions =

    [<Extension>]
    static member GetSlice(source: FlatList<'t>, from: int option, ``to``: int option) =
        let from = from |> Option.defaultValue 0
        let ``to`` = ``to`` |> Option.defaultValue source.Length
        source
        |> Seq.skip from
        |> Seq.take (``to`` - from + 1)
        |> Seq.toFlatList
```