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array.c: speedup Array#unshift by using space in shared array

• array.c: speedup Array#unshift by using space in shared array.
  [Feature #6638]
  • when array owns its shared array (ARY_SHARED_NUM == 1), and there
    is enough space then try unshift values directly into shared
    array.
  • when resulting array is big (~>64 items) then make it shared with
    enough room for future #unshifts, and then insert into shared
    array.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@37584 b2dd03c8-39d4-4d8f-98ff-823fe69b080e