Stabilize the following methods of `Result` as const:
- `is_ok`
- `is_err`
- `as_ref`
Possible because of stabilization of #49146 (Allow if and match in constants).
This commit adds new lang items which will be used in AST lowering once
`QPath::LangItem` is introduced.
Co-authored-by: Matthew Jasper <mjjasper1@gmail.com>
Signed-off-by: David Wood <david@davidtw.co>