mu4e: contacts: use mu4e--contacts-set

We use to have mu4e--contacts-hash, with name mapping to rank; that was
needlessly complicated since most completion engine sort alphabetically, making
the rank totally irrelevant (in practice, it doesn't matter much from the
end-user pov).

Anyway, simplify that part, maintain a set of contacts without any rank (which
what the server delivers now).

Also update the _default_ mu4e-contact-process-function to filter out anything
with 'reply' since it's not very useful for autocompletion.

mu4e/mu4e-search.el

@@ -441,7 +441,7 @@ status, STATUS."
    ((looking-back "\\(from\\|to\\|cc\\|bcc\\|contact\\|recip\\):\\([a-zA-Z0-9/.@]*\\)" nil)
     (list (match-beginning 2)
           (match-end 2)
-          mu4e--contacts-hash
+          mu4e--contacts-set
           :exit-function
           #'mu4e--search-completion-contacts-action))
    ((looking-back "list:\\([a-zA-Z0-9/.@]*\\)" nil)