Stabilize the Saturating type (saturating_int_impl, gh-87920)

Also stabilizes saturating_int_assign_impl, gh-92354.

And also make pub fns const where the underlying saturating_*
fns became const in the meantime since the Saturating type was
created.

library/alloc/src/lib.rs

@@ -144,7 +144,6 @@
 #![feature(ptr_metadata)]
 #![feature(ptr_sub_ptr)]
 #![feature(receiver_trait)]
-#![feature(saturating_int_impl)]
 #![feature(set_ptr_value)]
 #![feature(sized_type_properties)]
 #![feature(slice_from_ptr_range)]

library/core/src/num/mod.rs

@@ -44,11 +44,10 @@ mod uint_macros; // import uint_impl!
 mod error;
 mod int_log10;
 mod nonzero;
-#[unstable(feature = "saturating_int_impl", issue = "87920")]
 mod saturating;
 mod wrapping;
 
-#[unstable(feature = "saturating_int_impl", issue = "87920")]
+#[stable(feature = "saturating_int_impl", since = "CURRENT_RUSTC_VERSION")]
 pub use saturating::Saturating;
 #[stable(feature = "rust1", since = "1.0.0")]
 pub use wrapping::Wrapping;