refactor: remove #![feature(const_impl_trait)]

The `const_impl_trait` feature was stabilized by
<rust-lang/rust#93827>.

Author: yvt

src/r3/src/lib.rs

@@ -3,7 +3,6 @@
 #![feature(const_precise_live_drops)]
 #![feature(const_fn_fn_ptr_basics)]
 #![feature(type_alias_impl_trait)]
-#![feature(const_impl_trait)]
 #![feature(const_trait_impl)]
 #![feature(const_mut_refs)]
 #![feature(cell_update)]

src/r3/src/tests.rs

@@ -10,7 +10,6 @@ pub(crate) macro doc_test(
 ignore_if_port_std_does_not_support_target!($head), "\n ",
 "#   #![feature(const_fn_fn_ptr_basics)]
  #   #![feature(const_refs_to_cell)]
- #   #![feature(const_impl_trait)]
  #   #![feature(const_trait_impl)]
  #   #![feature(const_mut_refs)]
  #   #![deny(unsafe_op_in_unsafe_fn)]

src/r3_core/src/lib.rs

@@ -24,7 +24,6 @@
 #![feature(maybe_uninit_slice)]
 #![feature(const_nonnull_new)]
 #![feature(const_result_drop)]
-#![feature(const_impl_trait)]
 #![feature(const_option_ext)]
 #![feature(const_ptr_offset)]
 #![feature(const_trait_impl)]