Implement #[alloc_error_handler]

This to-be-stable attribute is equivalent to `#[lang = "oom"]`.
It is required when using the alloc crate without the std crate.
It is called by `handle_alloc_error`, which is in turned called
by "infallible" allocations APIs such as `Vec::push`.

src/libstd/lib.rs

@@ -233,8 +233,9 @@
 // std is implemented with unstable features, many of which are internal
 // compiler details that will never be stable
 #![feature(alloc)]
-#![feature(allocator_api)]
+#![feature(alloc_error_handler)]
 #![feature(alloc_system)]
+#![feature(allocator_api)]
 #![feature(allocator_internals)]
 #![feature(allow_internal_unsafe)]
 #![feature(allow_internal_unstable)]