fix(core): remove #![feature(untagged_unions)]

Unions with `impl Copy` and `ManuallyDrop` fields have been stabilized
by [rust-lang/rust#77547][1] back in 2020, making enabling the feature
already unnecessary for us. A few more field types have been added to
the allowlist later by [rust-lang/rust#97995][2], which also removed the
feature as there was no intent to stabilize more field types.

[1]: rust-lang/rust#77547
[2]: rust-lang/rust#97995

Author: yvt

src/r3_core/src/lib.rs

@@ -32,7 +32,6 @@
 #![feature(const_trait_impl)]
 #![feature(const_ptr_write)]
 #![feature(core_intrinsics)]
-#![feature(untagged_unions)] // `union` with non-`Copy` fields
 #![feature(assert_matches)]
 #![feature(const_mut_refs)]
 #![feature(const_ptr_read)]