Make ucs-names a hash table (Bug#28302)

Make ucs-names a hash table (Bug#28302)

* etc/NEWS: Mention the type change.
* lisp/descr-text.el (describe-char): Use gethash to access ucs-names.
Hardcode BEL's name into the function instead of needlessly mapping
over the hash table in the spirit of rassoc.
* lisp/international/mule-cmds.el (ucs-names): Fix variable and
function docstrings.  Initialize a hash table for ucs-names--the
number of entries is 42845 here.  Switch to hash-table
getters/setters.
(mule--ucs-names-annotation): Use hash-table getter.
(char-from-name): Upcase the string if ignore-case is truthy.
* lisp/leim/quail/latin-ltx.el: Use maphash instead of dolist.