<tuple>: tuple_cat for tuple-like types

[ArtemSarmini]

Description

Closes #260. I also replaced recursive impl of _View_as_tuple for arrays with index_sequence.

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[CaseyCarter]

This doesn't actually implement #260 since we use _STD get<_Kx> on line 951 which only supports array, pair, and tuple. I think we need to arrange for that to be the qualified std::get for standard types and unqualified get for non-standard types.

[BillyONeal]

Hi there, @ArtemSarmini are you still working on this?

[azure-pipelines]

Commenter does not have sufficient privileges for PR 461 in repo microsoft/STL

[azure-pipelines]

Pull request contains merge conflicts.

[StephanTLavavej]

It looks like @ArtemSarmini hasn't had time to look into this for the last couple of years - totally understandable, since the maintainer team was also incredibly busy! 😹 I'd like to ensure that the original contributor gets credit for looking into this issue, even if they've moved on to other things, so I'll pick up this PR (which is now our oldest active PR and the only triple-digit one). I believe the main issues are:

• Test coverage is needed (as the PR was created before we added the test harness, I believe),
• When I filed <tuple>: tuple_cat() should be generalized beyond pair and array #260 (from my personal todo list of "stuff that could be improved"), I wasn't thinking too hard about the consequences of enlarging our support for a non-Standard extension - which is something that we generally try to avoid these days. I was mostly thinking about simplifying our existing support for pair and array.
• There is now a proposal being reviewed by Library Evolution, P2165R3 "Compatibility between tuple, pair and tuple-like objects", that is highly relevant. In the latest revision R3, it has reduced its scope to types with std::get. Although this is not part of the Working Draft, I think this is a good reason to limit any rework here to only using std::get, instead of trying to accept get as a customization point far in advance of the Standard coming up with a design in this area.

Thus I believe that different code changes are needed, and will force-push this PR to start from today's main.

[azure-pipelines]

Commenter does not have sufficient privileges for PR 461 in repo microsoft/STL

[StephanTLavavej]

Actually, the Azure Pipelines bot is acting weird here; I will instead create a fresh PR and use Co-authored-by for crediting.