improve a performance in Array#hash

* before
      user     system      total        real
  1.350000   0.010000   1.360000 (  1.356039)

* after
      user     system      total        real
  0.800000   0.000000   0.800000 (  0.806837)

Test Code
----
Benchmark.bm do |x|
  ary = ["a", "b", "c"] * 10000

  x.report do
    1000.times do
      ary.hash
    end
  end
end