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

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

Contrast with String#length:

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