add String#byteslice() method

Text copied from http://redmine.ruby-lang.org/issues/4447. See that bug for details and MRI patch.

Please add a String#byteslice() method to the Ruby 1.9 core API.

Without that method, I am forced to inefficiently perform byte-based string slicing by (1) unpacking the entire string into an Array (with String#unpack or worse: my_string.bytes.to_a) then (2) slicing that Array and finally (3) joining the sliced Array into a string (with Array#pack or worse: my_array.map(&:chr).join), all as shown below:

class String
  unless method_defined? :byteslice
    ##
    # Does the same thing as String#slice but
    # operates on bytes instead of characters.
    #
    def byteslice(*args)
      unpack('C*').slice(*args).pack('C*')
    end
  end
end

Thanks for your consideration.