library: Move unstable API of new_uninit to new features

- `new_zeroed` variants move to `new_zeroed_alloc`
- the `write` fn moves to `box_uninit_write`

The remainder will be stabilized in upcoming patches, as
it was decided to only stabilize `uninit*` and `assume_init`.

compiler/rustc_index/src/lib.rs

@@ -2,6 +2,7 @@
 #![cfg_attr(all(feature = "nightly", test), feature(stmt_expr_attributes))]
 #![cfg_attr(feature = "nightly", allow(internal_features))]
 #![cfg_attr(feature = "nightly", feature(extend_one, new_uninit, step_trait, test))]
+#![cfg_attr(feature = "nightly", feature(new_zeroed_alloc))]
 // tidy-alphabetical-end
 
 pub mod bit_set;