[simple] Use #![feature(min_specialization)] instead of #![feature(specialization)]

This is a smaller and safer subset of `specialization`. It shouldn't cause any compiler
errors or undefined behavior. It'll silence our warning about specialization being "incomplete".

Author: Techcable

libs/simple/src/lib.rs

@@ -9,7 +9,7 @@
     const_panic, // Const panic should be stable
     untagged_unions, // Why isn't this stable?
     new_uninit, // Until Rust has const generics, this is how we init arrays..
-    specialization, // Effectively required by GcRef :(
+    min_specialization, // Effectively required by GcRef :(
 )]
 #![allow(
     /*