Feature request: Allow `url_*` functions to work with request objects

[billdenney]

It is currently possible to modify a URL in a request by extracting it, modifying it, and putting it back. It would be simpler for the user if request objects could be used as a URL in any relevant function.

Specifically, I think that it would apply for:

• url_modify
• url_modify_query (this could be a simple call to req_url_query which already calls to url_modify_query)
• url_modify_relative (this could be a simple call to url_modify_relative which already calls to url_modify_relative)
• url_parse
• url_query_parse

[billdenney]

If this is of interest, I'd be happy to make a PR.

[hadley]

I think I'd prefer to keep the req_ prefix for modify requests.