Fix the types in one of the FromResidual implementations

[scottmcm]

Without this change, ? on Result doesn't actually work as intended.

The compiler error from the new test I added, before fixing the types:

error[E0277]: the `?` operator can only be used in a function that returns `Result` or `Option` (or another type that implements `FromResidual`)
   --> core\lib\src\outcome.rs:665:25
    |
664 | /         fn demo() -> Outcome<i128, String, f32> {
665 | |             Err("hello")?;
    | |                         ^ cannot use the `?` operator in a function that returns `outcome::Outcome<i128, std::string::String, f32>`
666 | |             unreachable!()
667 | |         }
    | |_________- this function should return `Result` or `Option` to accept `?`
    |
    = help: the trait `FromResidual<std::result::Result<Infallible, &str>>` is not implemented for `outcome::Outcome<i128, std::string::String, f32>`
    = note: required by `from_residual`

And while I was there, I tweaked the code to remove the need for some #[allow]s.

[SergioBenitez]

Ah, that's interesting; I assumed the std-lib impl used !, but that's clearly not the case. Thanks for catching this.

[SergioBenitez]

Could you add a test for Outcome propagation, forward and failure, as well? Ideally we'd check that the variant and values match.

[scottmcm]

Hopefully the current work to stabilize ! will make it finally stick, and then it will, but for now it doesn't. (The plan is for Infallible to become a type alias for ! at that point, so people don't have to migrate.)

Context for why, if you're curious: rust-lang/rust#84092 (comment)

[scottmcm]

Added additional tests.

[SergioBenitez]

Awesome. Merged in f2a56f4. Thank you!