Bug: `TaskSeq.item` and `tryItem` should NOT read past the found item, not even a little bit

[abelbraaksma]

Consider this:

task {
    let mutable i = 0

    let ts = taskSeq {
        i <- i + 1
        yield 1
        i <- i + 1
        yield 2
        i <- i + 1 // we should never get here, if we pick idx 1
    }

    do! ts |> TaskSeq.item 1
    printfn "%i" i
}

This prints "3", not "2". That's wrong. Side effects beyond the item found should not execute.

[abelbraaksma]

Test added in #61.