(Q_superscript_of): New variable.

(Q_superscript_of): New variable.
(put_char_composition): Return `<-decomposition@superscript' instead
of `=decomposition' for compatibility mapping to single character
tagged `super'.
(Fput_char_attribute): Convert char-specs in value of
`{<-|->}superscript[^*]*' to characters and put reverse links.
(Fsave_char_attribute_table): Use `Fchar_refs_simplify_char_specs' for
`<-superscript'.
(syms_of_chartab): Add new symbol `<-superscript'.