Returns an array of the codepoints in +self+;
each codepoint is the integer value for a character:

  'hello'.codepoints     # => [104, 101, 108, 108, 111]
  'тест'.codepoints      # => [1090, 1077, 1089, 1090]
  'こんにちは'.codepoints # => [12371, 12435, 12395, 12385, 12399]
