Failed build when running with coverage on stable Rust

[tgross35]

Both wayland-backend and wayland-client have a line like the following:

wayland-rs/wayland-backend/src/lib.rs

Line 46 in 1fc7814

#![cfg_attr(coverage, feature(coverage_attribute))]

This is a problem because if building a project with coverage will always try to enable this feature, which will fail if using a stable rustc. This should be changed so the coverage_attribute feature and coverage(...) attributes to go with it are not enabled unless compiled with nightly.

I think the easiest fix is probably to add a Cargo feature gate for nightly features and use it in place of #[cfg_attr(coverage, coverage(...))], as well as updating the spots where the feature is enabled.

[uglyoldbob]

I would also like to see this implemented.