I seem to have missed the fact that `url-methods' populates the option
`url-proxy-services' from environment variables when initializing
`url-scheme-registry'. As explained in the docs, it actually accepts
full URLs instead of just host:port pairs. I take this to mean it's
probably less disruptive than initially thought to extend this liberty
to `url-proxy-services' itself. I've updated the POC patches to reflect
this, so it should now be possible to do:

    (setenv "HTTPS_PROXY" "socks5h://localhost:9050")
    (eww "https://check.torproject.org/")

Of course, this shouldn't interfere with traditional http proxies, such
as those that provide CONNECT tunneling:

    $ ncat -l --proxy-type http localhost 8888
    (setenv "HTTPS_PROXY" "localhost:8888")
    (eww "https://www.example.com/")