Add and_then combinator

[hlb8122]

Motivation
https://docs.rs/futures/0.3.8/futures/future/trait.TryFutureExt.html#method.and_then is a useful method on futures. Perhaps it'd be nice to replicate this for the ServiceExt API.

TODO

• Documentation
• Are the generics sane?

[hlb8122]

@LucioFranco Just in case you missed this discussion: hlb8122#1

[hawkw]

Overall, this looks good! I had one suggestion around error types.

Also, it would be nice to add a ServiceBuilder method for pushing an and_then layer, similar to the ones for other combinators.

[hawkw]

overall, this looks good. let's change the response future type to a newtype.

i also had a small style suggestion, but you can take it or leave it.

[hawkw]

let's change this to a named type re-exported from util::future, the way the other combinators do. I added a macro to make this easier --- if you rebase onto master, you can follow the example of PR #509

[hlb8122]

This doesn't seem possible:

opaque_future! {
    /// Response future from [`AndThen`] services.
    ///
    /// [`AndThen`]: crate::util::AndThen
    pub type AndThenFuture<F1, F2: TryFuture, N> = future::AndThen<future::ErrInto<F1, F2::Error>, F2, N>;
}

Is this acceptable instead?

opaque_future! {
    /// Response future from [`AndThen`] services.
    ///
    /// [`AndThen`]: crate::util::AndThen
    pub type AndThenFuture<F1, F2, F2Error, N> = future::AndThen<future::ErrInto<F1, F2Error>, F2, N>;
}

EDIT: I could also just write the top one without using the macro.

[hawkw]

Either is fine --- you're right that the macro doesn't support trait bounds currently. We could try to hack that in, but I think it would be fine to manually write our own newtype.

Remember that if you do that, you should write pub struct AndThenFuture<...>(...) and write a Future impl for it, rather than pub type AndThenFuture.... This is what the macro would expand to.

[hlb8122]

How's that look?

[hawkw]

looks good to me, thank you!