url: const-ify APIs, and pass URL by ref

Fixes warnings by Coverity Scan of inefficiences when passing by value
instead of passing by const reference.

PR-URL: https://github.com/nodejs/node/pull/15615


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>