Returns the count of characters (not bytes) in +self+:

  'foo'.length        # => 3
  'тест'.length       # => 4
  'こんにちは'.length   # => 5

Contrast with String#bytesize:

  'foo'.bytesize        # => 3
  'тест'.bytesize       # => 8
  'こんにちは'.bytesize   # => 15

