(Q_subscript_of): New variable.

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