Preserve static parts in dynamic container descriptors

[gldubc]

This is an updated version of #15275, rebased on current main, with follow-up refinements.

Closes #15273.

[gldubc]

I slightly modified the tuple emptiness algorithm while implementing this feature.

1. we perform only a structural check for empty elements when building a tuple, to avoid recursive calling of empty?
2. adjusted the way intersection is computed, to make sure no "type which is equivalent to none() but not exactly @none" can slip un-noticed

I think it's good now

[josevalim]

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