(Fget_composite_char): Use `Fchar_feature' instead of

(Fget_composite_char): Use `Fchar_feature' instead of
`Fget_char_attribute'.
(Fchar_variants): Likewise.
(get_char_table): Likewise.
(Fchar_feature): New function.
(put_char_composition): Use `Fchar_feature' instead of
`Fget_char_attribute'.
(Fput_char_attribute): Likewise; don't put the target character into
its `->unified' value.
(syms_of_chartab): Add new builtin function `char-feature'.