Split trait into `http-body-core` crate

[Kestrer]

The http-body crate has been gradually accumulating more utilities in it. This is a good thing, but it runs into the problem that breaking changes to any one of these utilities can cause a major ecosystem breakage, since even if trait Body defined in http-body 0.4 and 0.5 are the exact same trait signature-wise, Rust still treats them as entirely separate traits and will not allow them to interact. This also results in very confusing error messages that usually say something like "expected http_body::Body, found http_body::Body".

To solve this, this crate can be split into two (à la tower-service/tower-layer/tower, rand-core/rand and futures-core/futures-io/futures-sink/futures): http-body-core that contains only the definition of the trait itself, and http-body that contains BodyExt and all the useful utilities. With this design, if we were to release http-body v0.5 that contains breaking changes to the utilities but not the trait, it could re-export the old trait from the same version of http-body-core and there would be basically no breakage at all - both versions of http-body would be able to seamlessly coëxist and interoperate.

[seanmonstar]

That's indeed the objective of this crate: to just be the body trait hyper uses. I'd be more inclined to move the extra utilities into hyper, or an http-body-util crate.

[Kestrer]

The advantage of the http-body-core approach is that it wouldn't need breaking changes. But if you're fine with making them then that would definitely work as well.

[davidpdrsn]

I'm fine with splitting into http-body-core and http-body but I don't think the utilities should live in hyper. Then crates like tower-http would have to depend directly on hyper which makes it less useful imo. Arguably people using tower-http often use hyper anyway but would like to at least keep the option open.

[seanmonstar]

It's fine to make a util crate. My original goal of this crate was to just be the trait and nothing else, so people can safely require types from http and bound by http-body.