Implement IntoUrl for &Url

[jorendorff]

This was previously discussed in #412. Feel free to just close this, and accept my apology for wasting your time in that case. I just think maybe the reasoning there should be reconsidered.

In the general case, I agree: functions that need e.g. an owned String should just take a String argument. What makes this case special is that there's already a tacit admission that convenience matters here, i.e. IntoUrl existing and already accepting multiple types that require copying strings and parsing.

It's true that supporting &Url makes it possible for user code to do an extraneous clone unwittingly, but that's common in Rust. On the other hand, rejecting &Url makes it less convenient to use the more appropriate type, encouraging format! and Strings instead. On the whole I imagine this makes the world's code a little worse? Who can say.

On the third hand, we are most likely about to do a network request. Presumably that swamps the overhead of copying a single string. Maybe it's best to optimize for convenience here. Thanks for hearing me out and thanks for reqwest.

[jorendorff]

In case it's not obvious, this would be nice anywhere something could go wrong after reqwest's part is done, and the url is then wanted for the error:

let response = reqwest::get(&url).await?.error_for_status()?.json().await?;
if response.action == "jump" && response.how_high.is_none() {
    return Err(MyApiError::new(url, "missing how_high field in response"));
}