Use wrappers in the nonzero crate so users don’t need unstable features

components/compositing/Cargo.toml

@@ -9,9 +9,6 @@ publish = false
 name = "compositing"
 path = "lib.rs"
 
-[features]
-unstable = ["nonzero/unstable"]
-
 [dependencies]
 euclid = "0.15"
 gfx_traits = {path = "../gfx_traits"}

components/compositing/lib.rs

@@ -3,7 +3,6 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 #![deny(unsafe_code)]
-#![cfg_attr(feature = "unstable", feature(nonzero))]
 
 extern crate euclid;
 extern crate gfx_traits;