Allow users to create/use tuples

[StachuDotNet]

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As of #2545 and #4182, we have hidden support for Tuples - we don't allow users to create tuples, leaving the feature "turned off" via this switch

Here's work to be done with tuples:

• [x] add tuple stdlib fns #3309
• [x] hide tuples behind a feature flag rather than a constant value in-code (so we can use them in the meantime, and see what feels good/bad)
• [x] tuple match pattern: #4427
• [x] fluid bugfix: #4235
• [ ] fluid bugfix: #4264
• [ ] tuple destructuring #3402
• [ ] resolve all remaining TUPLETODOs
• [ ] #4239 (incl. backend type checking) maybe this should be split up?
• [ ] #4339
• [ ] remove the experimental setting - allow tuples by default
• [ ] #3310

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These should not block the general release of tuples, but are useful:

• [ ] #4475

[StachuDotNet]

I don't believe #4339 needs to be a blocker here, but I discovered the desire while writing a draft of docs updates.

[pbiggar]

I don't believe #4339 needs to be a blocker here, but I discovered the desire while writing a draft of docs updates.

I would say this should be done before we consider the project done.

[pbiggar]

Putting #3310 on your radar!

[StachuDotNet]

Putting #3310 on your radar!

Thanks, I thought it was already on the list! Adding.

[pbiggar]

So it is. I missed it when reading.

[StachuDotNet]

Ah right - forgot I put it in the footnote. Anyway, moved it to the 'main' section

[pbiggar]

I think we could enable to all users without #4475

[pbiggar]

Tuples are out