Auto merge of #76136 - CDirkx:const-result, r=dtolnay

Auto merge of #76136 - CDirkx:const-result, r=dtolnay

Stabilize some Result methods as const

Stabilize the following methods of Result as const:
 - `is_ok`
 - `is_err`
 - `as_ref`

A test is also included, analogous to the test for `const_option`.

These methods are currently const under the unstable feature `const_result` (tracking issue: #67520).
I believe these methods to be eligible for stabilization because of the stabilization of #49146 (Allow if and match in constants) and the trivial implementations, see also: [PR#75463](https://github.com/rust-lang/rust/pull/75463) and [PR#76135](https://github.com/rust-lang/rust/pull/76135).

Note: these methods are the only methods currently under the `const_result` feature, thus this PR results in the removal of the feature.

Related: #76225