Improve Accum function refactoring.

[ryukzak]

The problem you can find in the following test:

        , testCase "Complex items sum refactor" $
            let
                -- Start algorithm:
                -- tmp1, tmp2 = a + b
                -- tmp3, tmp4 = c + d
                -- res1 = one + tmp1 + tmp3
                -- res2 = two + tmp2 + tmp4
                -- res = res1 + res2

                -- Result algorithm:
                -- res = one + a + b + c + d + two + a + b + c + d
                func1 = acc [Push Plus (I "a"), Push Plus (I "b"), Pull (O $ S.fromList ["tmp1", "tmp2"])]
                func2 = acc [Push Plus (I "c"), Push Plus (I "d"), Pull (O $ S.fromList ["tmp3", "tmp4"])]
                func3 = acc [Push Plus (I "one"), Push Plus (I "tmp1"), Push Plus (I "tmp3"), Pull (O $ S.fromList ["res1"])]
                func4 = acc [Push Plus (I "two"), Push Plus (I "tmp2"), Push Plus (I "tmp4"), Pull (O $ S.fromList ["res2"])]
                func5 = acc [Push Plus (I "res1"), Push Plus (I "res2"), Pull (O $ S.fromList ["res"])]

                funcRes =
                    acc
                        [ Push Plus (I "one")
                        , Push Plus (I "a")
                        , Push Plus (I "b")
                        , Push Plus (I "c")
                        , Push Plus (I "d")
                        , Push Plus (I "two")
                        , Push Plus (I "a")
                        , Push Plus (I "b")
                        , Push Plus (I "c")
                        , Push Plus (I "d")
                        , Pull (O $ S.fromList ["res"])
                        ] ::
                        F String Int
             in
                [func1, func2, func3, func4, func5] `refactorTo` [funcRes]

    Complex items sum refactor:                                                FAIL
      test/NITTA/Model/Problems/Refactor/Accum/Tests.hs:155:
      expected: fromList [Acc(+one +a +b +c +d +two +a +b +c +d = res)]
       but got: fromList [Acc(+a +b = tmp1 = tmp2),Acc(+c +d = tmp3 = tmp4),Acc(+one +tmp1 +tmp3 +two +tmp2 +tmp4 = res)]
      Use -p '/Complex items sum refactor/' to rerun this test only.

As you see, after refactoring we have a lot of buffer variables. They should be merged in one function to reduce numbers of transactions.