std: Stabilize default

All stable.

Author: brson

src/libcore/default.rs

@@ -10,6 +10,8 @@
 
 //! The `Default` trait for types which may have meaningful default values
 
+#![stable]
+
 /// A trait that types which have a useful default value should implement.
 pub trait Default {
     /// Return the "default value" for a type.