librustc: Put #[unsafe_destructor] behind a feature gate.

Closes #8142.

This is not the semantics we want long-term. You can continue to use
`#[unsafe_destructor]`, but you'll need to add
`#![feature(unsafe_destructor)]` to the crate attributes.

[breaking-change]

src/liballoc/lib.rs

@@ -69,7 +69,8 @@
        html_root_url = "http://doc.rust-lang.org/")]
 
 #![no_std]
-#![feature(phase)]
+#![feature(phase, unsafe_destructor)]
+#![allow(unknown_features)] // NOTE: remove after a stage0 snap
 
 #[phase(plugin, link)]
 extern crate core;