zlib: instance-ify two methods

Both `Params` and `SetDictionary` take `ZCtx` as an arg, so just make
them both instance methods on `ZCtx`.

PR-URL: https://github.com/nodejs/node/pull/21702


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>