libstd: Add unwrap_or and unwrap_or_handle to Result

[Ryman]

It might make more sense to mirror Option's unwrap_or_else but I've left it as handle as it feels more explicit about the signature difference.

[brson]

I believe the current idea with the Option/Result interfaces is that for things like unwrap_or you can just convert to an Option with .ok(). unwrap_or_handle though looks Result-specific. Perhaps we can leave out unwrap_or and just have unwrap_or_handle.

[brson]

Oh, hm. Result does define unwrap which could trivially be done with ok().unwrap() so I guess it's not so clear cut.

Does anybody remember why we kept unwrap? Is it the potential for more specific error messages?

[brson]

Well, I r+'d based on existing standards.

[sfackler]

@brson I think Result::unwrap is around from when we used to require that the error type be Show. It's superfluous now. I'd very much like to have Result::unwrap only defined for E: Show and have it actually add the error to the failure message. ok().unwrap() is an easy fallback for non-showable error types.

[brson]

@sfackler Yeah that sounds reasonable. I don't recall why we removed the Show bound.

[sfackler]

In the old implementation, all methods required a Show bound, which was kind of excessive. I'm writing up a PR right now.

[Ryman]

@sfackler : That should fix #13379 👍