refactor
loop unrolling to improve the Objective-C methods performance in String object

* before
      user     system      total        real
  2.310000   0.320000   2.630000 (  3.078306)

* after
      user     system      total        real
  1.560000   0.230000   1.790000 (  2.309584)

Test Code:
----
framework 'Foundation'
require 'benchmark'

Benchmark.bm do |x|
  str = "abc" * 100000000
  x.report do
    str.writeToFile("test.txt", atomically:true, encoding:NSUTF8StringEncoding, error:nil)
  end
end