Returns the successor to +self+. The successor is calculated by
incrementing characters.

The first character to be incremented is the rightmost alphanumeric:
or, if no alphanumerics, the rightmost character:

  'THX1138'.succ   # => "THX1139"
  '<<koala>>'.succ # => "<<koalb>>"
  '***'.succ       # => '**+'
  'тест'.succ      # => "тесу"
  'こんにちは'.succ  # => "こんにちば"

The successor to a digit is another digit, "carrying" to the next-left
character for a "rollover" from 9 to 0, and prepending another digit
if necessary:

  '00'.succ # => "01"
  '09'.succ # => "10"
  '99'.succ # => "100"

The successor to a letter is another letter of the same case,
carrying to the next-left character for a rollover,
and prepending another same-case letter if necessary:

  'aa'.succ # => "ab"
  'az'.succ # => "ba"
  'zz'.succ # => "aaa"
  'AA'.succ # => "AB"
  'AZ'.succ # => "BA"
  'ZZ'.succ # => "AAA"

The successor to a non-alphanumeric character is the next character
in the underlying character set's collating sequence,
carrying to the next-left character for a rollover,
and prepending another character if necessary:

  s = 0.chr * 3   # => "\x00\x00\x00"
  s.succ        # => "\x00\x00\x01"
  s = 255.chr * 3 # => "\xFF\xFF\xFF"
  s.succ        # => "\x01\x00\x00\x00"

Carrying can occur between and among mixtures of alphanumeric characters:

  s = 'zz99zz99' # => "zz99zz99"
  s.succ         # => "aaa00aa00"
  s = '99zz99zz' # => "99zz99zz"
  s.succ         # => "100aa00aa"

The successor to an empty +String+ is a new empty +String+:

  ''.succ # => ""

Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].