Stabilize flatten/flatMap family of operators

[qwwdfsad]

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While it's still closely bound to #1147, it's worth investigating and draft-designing the original operators stabilization, so our flatMap analogue will be available for users who don't use any experimental API

[CoreFloDev]

Hi @qwwdfsad

I got the same question about the merge() and scan() operator, any blockers that don't make them suitable for stabilization?

Thanks!

[gabrielittner]

Any updates on stabilizing these operators?