Stabilize Result::as_deref and as_deref_mut

[tesuji]

FCP completed in #50264 (comment).

This PR stabilizes two methods for std::result::Result:

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 methods from Result:

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

[dtolnay]

Thanks!

[dtolnay]

@bors r+

[bors]

📌 Commit 68017c2d4ad7018696e56219bceb3d878801acb0 has been approved by dtolnay

[dtolnay]

@bors r-

Libcore tests need to be updated.

error[E0599]: no method named `as_deref_err` found for reference `&std::result::Result<u8, &i32>` in the current scope
   --> library/core/tests/result.rs:257:24
    |
257 |     assert_eq!(ref_err.as_deref_err(), expected_result);
    |                        ^^^^^^^^^^^^ method not found in `&std::result::Result<u8, &i32>`

[dtolnay]

Thanks!

[dtolnay]

@bors r+

[bors]

📌 Commit 6d293ed has been approved by dtolnay

[bors]

⌛ Testing commit 6d293ed with merge 19ecce3...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: dtolnay
Pushing 19ecce3 to master...