[Pitch] Introduce existential `any`

[ksluder (Kyle Sluder)]

@Ben_Cohen

I am not sure this needs to be in "alternatives considered"... the word considered here needs to mean actually considered. Not "alternatives that could possibly be envisioned".

I appreciate your greater point, but this seems like a mischaracterization. Any<P> has been considered to the extent of even getting a full Swift Evolution proposal, SE-0089

It was pitched again by @bjhomer last August.

And my memory of the state of conversation prior to @Joe_Groff’s codification in the “Improving UI” thread was mostly ambivalence toward Any<P> vs. any P.