Returns +true+ if +self+ and +object+ are equal after Unicode case folding,
otherwise +false+:

  lower = :abc
  upper = :ABC
  upper.casecmp?(lower) # => true
  lower.casecmp?(lower) # => true
  lower.casecmp?(upper) # => true

Returns nil if +self+ and +object+ have incompatible encodings,
or if +object+ is not a symbol:

  sym = 'äöü'.encode("ISO-8859-1").to_sym
  other_sym = 'ÄÖÜ'
  sym.casecmp?(other_sym) # => nil
  :foo.casecmp?(2)        # => nil

Unlike Symbol#casecmp, works for characters outside of 'A'..'Z' and 'a'..'z':

  lower = :äöü
  upper = :ÄÖÜ
  upper.casecmp?(lower) # => true
  lower.casecmp?(lower) # => true
  lower.casecmp?(upper) # => true

Related: Symbol#casecmp, String#casecmp?.