Stabilize wayland-backend

[notgull]

wayland-backend is planned to be used in the public API of winit, by way of wayland-csd-frame. Since it's a public dependency it would be nice if it was stable, i.e. released version 1.0.

I know that wayland-client, wayland-server and other dependents probably want to experiment more with their API and thus can't be released stably yet. However I think it would be nice to have wayland-backend as a stable, solid interface to wayland.

Potential semver hazards I can see:

• wayland-backend has wayland-sys (and therefore dlib), smallvec and downcast-rs as public dependencies. I think these dependencies would either need to be made private or made stable as well.
• Some of the fields in the protocol module could be made private instead.
• There should be a way of disabling the pure-Rust backend with features.

[kchibisov]

I'm not sure it's planned to be used in a public api of winit, but other than that making things more stable is welcome.

[ids1024]

I like the idea of making wayland-backend more stable. This was discussed a bit in #721.

Not sure how many challenges there are to this. #678 is a breaking change to wayland-backend. I think other breaking API changes that are planned currently are generally in higher level crates? If there are any other things that are likely to change, we can think about whether the public API of wayland-backend can be made more minimal.

wayland-backend has wayland-sys (and therefore dlib), smallvec and downcast-rs as public dependencies. I think these dependencies would either need to be made private or made stable as well.

Do we actually expose dlib anywhere in the API? But yeah, not exposing downcast-rs publicly is probably good. smallvec has been on version 1.x for a while, but apparently has 2.x alpha releases, so I guess that's a concern too.