Returns the count of bytes in +self+.

Note that the byte count may be different from the character count (returned by #size):

  s = 'foo'
  s.bytesize # => 3
  s.size     # => 3
  s = 'こんにちは'
  s.bytesize # => 15
  s.size     # => 5

Related: see {Querying}[rdoc-ref:String@Querying].
