Remove old features clone_from_slice and convert

[nox]

components/canvas/lib.rs:5:12: 5:28 warning: this feature is stable. attribute no longer needed, #[warn(stable_features)] on by default
components/canvas/lib.rs:5 #![feature(clone_from_slice)]
                                      ^~~~~~~~~~~~~~~~
components/gfx/lib.rs:6:75: 6:82 warning: unused or unknown feature, #[warn(unused_features)] on by default
components/gfx/lib.rs:6 #![cfg_attr(any(target_arch = "x86_64", target_arch = "aarch64"), feature(convert))]
                                                                                                  ^~~~~~~
components/compositing/lib.rs:6:12: 6:28 warning: this feature is stable. attribute no longer needed, #[warn(stable_features)] on by default
components/compositing/lib.rs:6 #![feature(clone_from_slice)]
                                           ^~~~~~~~~~~~~~~~

[timvandermeij]

I would like to work on this one.

[jdm]

Go for it!