Add a sealed::Sealed trait to core, like that in std

This allows traits in `core` to be sealed as well.

This could be the same trait as `std` via a re-export, but that would
require `core::sealed` to be `pub` (even if unstable). Keep them as
separate traits for now.

library/core/src/lib.rs

@@ -110,6 +110,7 @@
 #![feature(offset_of_enum)]
 #![feature(panic_internals)]
 #![feature(pattern_type_macro)]
+#![feature(sealed)]
 #![feature(ub_checks)]
 // tidy-alphabetical-end
 //
@@ -216,6 +217,14 @@ pub mod from {
     pub use crate::macros::builtin::From;
 }
 
+mod sealed {
+    /// This trait being unreachable from outside the crate
+    /// prevents outside implementations of our extension traits.
+    /// This allows adding more trait methods in the future.
+    #[unstable(feature = "sealed", issue = "none")]
+    pub trait Sealed {}
+}
+
 // We don't export this through #[macro_export] for now, to avoid breakage.
 #[unstable(feature = "autodiff", issue = "124509")]
 /// Unstable module containing the unstable `autodiff` macro.