Created originally on Bitbucket by kostialopuhin (Konstantin Lopukhin)
Was already merged in Bitbucket before import, marked as merged by the import user
Instead of building result by doing inplace addition of string, make a list of pieces and join them together. Before this fix pypy was way slower then cpython on long strings. Now it is about 2 x faster on long strings, and 4-5 x faster on small strings.