Unify characters of CHARSET with Unicode.
This means reading the relevant file and installing the table defined
by CHARSET's `:unify-map' property.
Optional second arg UNIFY-MAP is a file name string or a vector. It has
the same meaning as the `:unify-map' attribute in the function
`define-charset' (which see).
Optional third argument DEUNIFY, if non-nil, means to de-unify CHARSET.
(fn CHARSET &optional UNIFY-MAP DEUNIFY)