feat(util): introduce TryFutureBody<F, B>

[cratelyn]

fixes #157.

this commit introduces a new utility to http-body-util, permitting
callers to treat a fallible Future<Output = Result<Body, _>> as a
body. while the future is still pending, the inner future will be
polled. once the future yields a body, the body will then be polled for
its contents.

one important difference in the code in this commit as compared to the
original snippet proposed in #157 is that if the future fails and yields
an error, the error will be propagated and the body will be marked as
having failed.

like the Either<L, R> middleware, this adapter type works around some
minor limitations in pin-project-lite. namely, enum tuple variants are
not supported, and doc-comments (required here per the missing_docs
lint enforced in this library) also were not parsed properly.

a proj submodule contains code derived from the output generated by
the pin_project! macro, with some additional commentary added to be
thorough about noting safety with respect to Pin<T> projection.

a small test suite is included to show that error propagation works as
expected, hints work correctly, and that data from the inner body is
returned correctly.

one other detail about the code in this commit worth calling out is
that this middleware is named TryFutureBody<F, B> rather than
FutureBody<F, B>. because this works with futures whose output is a
fallible Result<B, E>, it felt like a forward-compatible choice to
choose this name instead. that will permit the future addition of a
FutureBody<F, B> that wraps futures that emit a plain B body. that
is not included in this commit, so as to facilitate review, but can be
added as a simple follow-up to this proposal.

[seanmonstar]

Reminds me of tower's FutureService. Sounds good.

Perhaps it should be an opaque struct publicly, and the enum variants internal?