Remove rustc compiler features to unbreak rustc nightly build;

- following landing of rust-lang/rust#73858,
the `const_saturing_int_methods` feature is in a weird nonexisting
state (stabilized in a future Rust version), so remove it from the list
to unblock compilation.
- also remove other const features that have stabilized in the
meanwhile to avoid compiler warnings.

src/lib.rs

@@ -31,14 +31,11 @@
   const_fn,
   const_fn_union,
   const_generics,
-  const_if_match,
-  const_loop,
   const_mut_refs,
   const_panic,
   const_ptr_offset_from,
   const_raw_ptr_deref,
   const_raw_ptr_to_usize_cast,
-  const_saturating_int_methods,
   const_trait_impl,
   core_intrinsics,
   doc_cfg,