impl Transform for Option

This topic continues https://github.com/actix/actix-web/pull/2858 , whole goal is to `.wrap` an optional middleware. `Condition` middleware exists but requires a transformer even when unused. It seems to me that using an `Option` is more natural. But as that PR was on a different crate than `Transform`'s, a newtype was introduced, and `.into()` required an explicit annotation. Sadly this was non-optimal.

@fakeshadow suggested that this feature should rather be moved into `Transform`'s crate, so that `.wrap` can natively take an `Option`. This would mean moving the `ConditionMiddleware` code too.