fix: add #![feature(const_maybe_uninit_uninit_array)]

The `const`-ness of `MaybeUninit::uninit_array` has been split into a
separate feature by [rust-lang/rust#96099][1].

[1]: rust-lang/rust#96099

Author: yvt

src/r3_core/src/lib.rs

@@ -1,5 +1,6 @@
 #![feature(const_fn_floating_point_arithmetic)]
 #![feature(const_nonnull_slice_from_raw_parts)]
+#![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(nonnull_slice_from_raw_parts)]

src/r3_kernel/src/lib.rs

@@ -1,3 +1,4 @@
+#![feature(const_maybe_uninit_uninit_array)]
 #![feature(const_maybe_uninit_assume_init)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(maybe_uninit_uninit_array)]