(Q_identical): New variable.

(Q_identical): New variable.
(Q_identical_from): New variable.
(Fchar_feature): Regard `->identical' as a feature to find ancestors.
(Fput_char_attribute): Add reversed link for `{->|<-}identical'.
(Fsave_char_attribute_table): Use `Fchar_refs_simplify_char_specs' for
`{->|<-}identical'.
(syms_of_chartab): Add new symbols `{->|<-}identical'.