Remove Pin<Box<dyn Future>> from FromRequest implementations

[hlbarber]

At the time of writing FromRequest::Future implementations are Pin<Box<dyn Future>>. By hand crafting a Future to get a concrete type we should be able to remove this heap allocation. This might involve rewriting https://docs.rs/hyper/latest/hyper/body/fn.to_bytes.html to give it a named type.

Support for async fn in traits might also be sufficient.

[hlbarber]

Preemptively newtyping Pin<Box<dyn Future>> would also reduce breakage in the future, even without the rewrite of to_bytes.

[hlbarber]

As part of the migration from the old pre-1.0 hyper ecosystem, the HttpBody trait has evolved to a new, more versatile interface, this includes a rewrite of the surrounding utilities.

https://docs.rs/http-body-util/0.1.0-rc.2/http_body_util/combinators/struct.Collect.html

Context can be found here hyperium/hyper#3111 (comment)

This is a named Future which we can use to construct the associated type.