Implement FromStr for Url

[wycats]

This was supported in the old url package. We used it to import env-var based configuration into a variety of types.

[SimonSapin]

I’ve resisted doing this so far because I really don’t like FromStr. It feels like "too much magic" to me: when reading code that uses it, I sometimes have no idea what the expected type is because the compiler infers it.

I’m not familar with that configuration system, how much worse does it make it for you if you have to write Url::parse(string).ok() instead?

[TyOverby]

Other libraries (like an argument parser library) could depend on an item having FromStr. I think it's a nice usability feature.