Warp: Expose CombineRejection in public API

[dylanede]

Without it, it is difficult to write helper functions that take filters as arguments and combine them with others, since warp built-in filter combinators often have bounds involving CombineRejection, which cannot be propagated to the signature of the helper function.

[aloucks]

It would be nice if all of the filters (e.g. And, AndThen, etc) were made public. Currently, the generated documentation makes it impossible to see what these things actually are.

[jxs]

@dylanede can you show what you are trying to achieve?
@aloucks see https://github.com/seanmonstar/warp/issues/607#issuecomment-650448029

[droundy]

I would like to be able to put some of my filters into a separate function, but I can't return the result of that function in such a way that they can be used, because I can't name the return type, and also can't use impl Trait, since the return type involves a closure, and the trait required is not public.