Auto merge of #74948 - lzutao:stalize-result-as-deref, r=dtolnay

Stabilize `Result::as_deref` and `as_deref_mut`

FCP completed in https://github.com/rust-lang/rust/issues/50264#issuecomment-645681400.

This PR stabilizes two new APIs for `std::result::Result`:
```rust
fn as_deref(&self) -> Result<&T::Target, &E> where T: Deref;
fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E> where T: DerefMut;
```

This PR also removes two rarely used unstable APIs from `Result`:
```rust
fn as_deref_err(&self) -> Result<&T, &E::Target> where E: Deref;
fn as_deref_mut_err(&mut self) -> Result<&mut T, &mut E::Target> where E: DerefMut;
```

Closes #50264

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