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`.

library/std/src/lib.rs

@@ -363,6 +363,7 @@
 #![feature(get_mut_unchecked)]
 #![feature(map_try_insert)]
 #![feature(new_uninit)]
+#![feature(new_zeroed_alloc)]
 #![feature(slice_concat_trait)]
 #![feature(thin_box)]
 #![feature(try_reserve_kind)]