Support for importing artefacts from another flow expression using "import" and "take" operations

[msdvoits]

let ``Extend and continue work using computation expression``() =
    let w1 = work {
        let! x = makeValue 1 |> name "one"
        let! y = inc x
        let! z = inc x 
        let! w = add(x,y) |> name "sum"
        ()
    }
    let eng,names,annotations,_ = startFrom_x w1 (createDefaultSuffix)
    let wrk = Execution.EngineAPI.AwaitCompletion eng |> Async.RunSynchronously
    eng.Dispose()

    Trace.WriteLine("\n\n\nContinues...")
    let snapshot = wrk, names, annotations
    let w2 = work {
        let! y = snapshot |> take<int> "out one"
        let! w = snapshot |> take<int> "out sum"
        let! z = inc y
        return z
    }

    let (g,_,_) = build w2 
    Assert.AreEqual(4, g.Structure.Vertices.Count)

    match eval w2 |> Async.RunSynchronously with
    | EvaluationResult.Failed _ -> failwith "Work failed"
    | EvaluationResult.Success z -> 
        z =? 3