[Pitch] Introduce existential `any`

[anon9791410]

@hborla

the transition to the new syntax is mechanical, so it can be performed automatically by a migrator.

That must mean that the as Protocols turn into as any Protocols, yes?

If so, we can't reasonably have support for as any without as some as well. If the latter comes along with the former, this is a doubly great pitch.