Migrate `next_*` methods to return `Option`

[frewsxcv]

In the Rust standard library, the next method on the Iterator trait
returns an Option. Prior to this commit, the next and next_*
methods return Err<_, ()>. In an attempt to make some of the code more
idiomatic, I migrated these methods to return Option instead.

[SimonSapin]

Thanks for the good intentions, but using Result<_, ()> instead of Option<_> was very deliberate. It enables using the try! macro, which I do extensively in Servo.

I’ve experimented with a generalized try! macro that works on Option as well as Result, but I’m not very happy with it since it doesn’t convert between them.

Until returning Option can be as ergonomic, I prefer sticking with Result.

[frewsxcv]

since it doesn’t convert between them

What do you mean by this?

[SimonSapin]

I mean that you can use this modified try! macro with an Option<T> expression in a context where the return type is Option<U>, or with a Result<T, E> expression where it is Result<U, E>, but not e.g. with a Option<T> expression where the return type is Result<U, E>.