feat(http): impl `IntoFuture` for requests, deprecate `exec()`

[vilgotf]

Depends upon #1897.

[vilgotf]

Can the clippy lint be ignored? I think it's complaining about 0.14 not being defined (current Twilight version is 0.13). Should this PR bump all crates to 0.14?

[7596ff]

I think it wants you to say 0.14.0

[vilgotf]

what do you think of putting this in response::future

pub(crate) type ResponseFutureOutput<T> = Result<Response<T>, Error>;

I'm generally not a big fan of types as they, without an IDE, can be irritating to resolve (especially private one's as rustdoc doesn't make them clickable).