Add is and as support for std::expected

[bluetarpmedia]

• Update cpp2util.h to support is and as for std::expected
• Add new pure2-expected-is-as regression test

[hsutter]

Thanks! @filipsajdak @bluetarpmedia Does this conflict with #701, which I think is close to ready to review again?

[filipsajdak]

There is a conflict. I can prepare an update to it. But first I will split #701.

[bluetarpmedia]

There is a conflict. I can prepare an update to it. But first I will split #701.

Thanks @filipsajdak -- I didn't mean to intrude on your work!

[filipsajdak]

No intrusion. I like that you bring up the std::expected

[bluetarpmedia]

I wasn't able to successfully rebase this onto the updated code in main, so I've closed this and created #971.

[JohelEGP]

You can always take the local branch that you updated
and do git checkout this_branch, git reset --hard that_branch, and git push -f
to avoid closing and reopening a PR.