Auto merge of #44015 - kennytm:hasher, r=alexcrichton

 impl Hasher for {&mut Hasher, Box<Hasher>}

**Rationale:** The `Hash` trait has `fn hash<H: Hasher>(&self, state: &mut H)`, which can only accept a `Sized` hasher, even if the `Hasher` trait is object-safe. We cannot retroactively add the `?Sized` bound without breaking stability, thus implementing `Hasher` to a trait object reference is the next best solution.

**Warning:** These `impl` are insta-stable, and should need an FCP. I don't think a full RFC is necessary.

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