util: Add Either util

[indietyp]

This PR introduces a new type, Either, as discussed in #59.

Either implements:

• std::error::Error
• bytes::Buf
• http_body::Body
• From<Result<L, R>>

Either also provides some methods like: flip(), map_left(), map_right(), map(), either(), as_ref(), into_inner().

Due to the limitations of pin-project-lite this PR does not use the pin_project! macro but instead uses a slightly adjusted expanded version.

This change is entirely backward compatible.

[davidpdrsn]

This just my opinion but I don't think we need all the other impls, just Body. I think users are unlikely to need them. I've never needed them myself when using http-body.

Same goes for all the helper methods on Either. In my experience you just need an Either so you can return two different body types from your tower::Service. The body isn't consumed as an Either but as a generic B: Body so all the additional methods on Either aren't necessary.

[indietyp]

I see. Currently, this implements Either for Body, Error, and Buf. I thought that maybe - at least for Error it'd be helpful to keep track of the error on both sides instead of restricting the error to a single type. I understand removing Either on Data - as it is unlikely that you work with two different data streams, but I am not so sure about Error, what do you think?

[davidpdrsn]

I think we should use Box<dyn Error + Send + Sync> as the error type, similar to what tower does. You can read about the reasons why here.

[indietyp]

Thanks for the hint. That makes a lot of sense. I have implemented your suggestions.

[LucioFranco]

Cool, I think if you update my one nit then we can merge this. Thanks!

[indietyp]

Awesome 🎉, I am probably going to be able to apply your suggestions by tomorrow.