Rollup merge of #99736 - lopopolo:lopopolo/gh-80996-partial-stabilization-bounds-as-ref, r=dtolnay

Partially stabilize `bound_as_ref` by stabilizing `Bound::as_ref`

Stabilizing `Bound::as_ref` will simplify the implementation for `RangeBounds<usize>` for custom range types:

```rust
impl RangeBounds<usize> for Region {
    fn start_bound(&self) -> Bound<&usize> {
        // TODO: Use `self.start.as_ref()` when upstream `std` stabilizes:
        // https://github.com/rust-lang/rust/issues/80996
        match self.start {
            Bound::Included(ref bound) => Bound::Included(bound),
            Bound::Excluded(ref bound) => Bound::Excluded(bound),
            Bound::Unbounded => Bound::Unbounded,
        }
    }

    fn end_bound(&self) -> Bound<&usize> {
        // TODO: Use `self.end.as_ref()` when upstream `std` stabilizes:
        // https://github.com/rust-lang/rust/issues/80996
        match self.end {
            Bound::Included(ref bound) => Bound::Included(bound),
            Bound::Excluded(ref bound) => Bound::Excluded(bound),
            Bound::Unbounded => Bound::Unbounded,
        }
    }
}
```

See:

- #80996
- https://github.com/rust-lang/rust/issues/80996#issuecomment-1194575470

cc `@yaahc` who suggested partial stabilization.

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