Stabilize the Saturating type (saturating_int_impl, rust-langgh-87920)

Also stabilizes saturating_int_assign_impl, rust-langgh-92354.

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

tests/ui/arithmetic_side_effects.rs

@@ -10,7 +10,7 @@
     arithmetic_overflow,
     unconditional_panic
 )]
-#![feature(const_mut_refs, inline_const, saturating_int_impl)]
+#![feature(const_mut_refs, inline_const)]
 #![warn(clippy::arithmetic_side_effects)]
 
 extern crate proc_macro_derive;