Rollup merge of #87769 - m-ou-se:alloc-features-cleanup, r=yaahc,dtolnay

Alloc features cleanup

This sorts and categorizes the `#![features]` in `alloc` and removes unused ones.

This is part of #87766

The following feature attributes were unnecessary and are removed:

```diff
// Library features:
-#![feature(cow_is_borrowed)]
-#![feature(maybe_uninit_uninit_array)]
-#![feature(slice_partition_dedup)]

// Language features:
-#![feature(arbitrary_self_types)]
-#![feature(auto_traits)]
-#![feature(box_patterns)]
-#![feature(decl_macro)]
-#![feature(nll)]
```

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