Improve ergonomics by allowing to use a string input for fragment selections

[svenstaro]

The intermediate parsing of the Selector itself is a bit annoying. I think it would be neat if it were part of the select().

Current:

let fragment = Html::parse_fragment(html);
let selector = Selector::parse("li").unwrap();

for element in fragment.select(&selector) {}

Proposed a):

let fragment = Html::parse_fragment(html);
for element in fragment.select("li".into()?) {}

Proposed b):

let fragment = Html::parse_fragment(html);
for element in fragment.select("li")? {}

[brokenthorn]

Isn't it more idiomatic Rust to implement Into<Selector> for str and/or String, instead of adding an extra function try_select, in #30 ?

[spegoraro]

@brokenthorn I agree. I think it makes it much clearer as to what's going on, however it's more idiomatic again to implement TryFrom<&str> for Selector. The From (and implied Into) traits are meant for conversions that won't fail. Using their Try* equivalents returns a Result which makes more sense. By implementing TryFrom we get TryInto for free also.

Note, that it would be awesome to do something even more generic like this: impl<'i, T: Deref<Target = str>> TryFrom<&'i T> for Selector so that we get deref coercion automatically but at the moment that will cause a conflict with the blanket implementation in core (see rust-lang/rust#50133 (comment)).