Returns a basic +n+-bit checksum of the characters in +self+;
the checksum is the sum of the binary value of each byte in +self+,
modulo <tt>2**n - 1</tt>:

  'hello'.sum     # => 532
  'hello'.sum(4)  # => 4
  'hello'.sum(64) # => 532
  'тест'.sum      # => 1405
  'こんにちは'.sum  # => 2582

This is not a particularly strong checksum.
