Add or_try_* variants for HashMap Entry API

[peterjoel]

These methods are technically unneeded as you can implement them yourself by matching on Entry::Occupied and Entry::Vacant, but this makes it more ergonomic to use the Entry API in cases where construction of a value can fail.

I realise that the same methods can also be added to BTreeMap's Entry, but I thought I'd wait to see if this was accepted first.

ACP: rust-lang/libs-team#336

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[Jules-Bertholet]

Should this use the Try trait?

[peterjoel]

@Jules-Bertholet You mean something like this?

pub fn or_try_insert_with<F, R>(self, default: F) -> R
where:
    F: FnOnce() -> R,
    R: Try<Output = V>,

[peterjoel]

Actually I think this level of genericity is not possible without HKT because the type of Try::Output is different in the return types of the default function and or_try_insert_with itself.

The default function can still have a generic return type, but I think or_try_insert_with's return type needs to be fixed as a Result.

pub fn or_try_insert_with<F, R>(self, default: F) -> Result<&'a mut V, R::Residual>
where
    F: FnOnce() -> R,
    R: Try<Output = V>,

[peterjoel]

My understanding of Try was out of date. In the try_trait_v2 implementation, the residual of, e..g, Result<T, E> is Result<!, E> (not E as I has assumed), and there doesn't appear to be a way to retrieve the E at either the type or value level.

@Jules-Bertholet Please let me know if I'm missing something, but I think my original signature is what we're stuck with, without larger changes to the error-handling traits.

[Noratrieb]

Thank you for the PR! This PR contains a public library API change, which should follow the proper process of creating an ACP first: https://std-dev-guide.rust-lang.org/development/feature-lifecycle.html
Use @rustbot ready when your ACP has been accepeted.
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[peterjoel]

Thanks @Nilstrieb, I thought previously this wasn't necessary for small changes, and didn't think to check! I'll take a look at that now.