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/std/src/lib.rs

@@ -351,7 +351,6 @@
 #![feature(get_many_mut)]
 #![feature(lazy_cell)]
 #![feature(log_syntax)]
-#![feature(saturating_int_impl)]
 #![feature(stdsimd)]
 #![feature(test)]
 #![feature(trace_macros)]