SE-0309: Unlock existential types for all protocols

[anthonylatsis]

@1oo7

Removing the new syntax any from the proposal will help this review focus on the core aspects of the proposal, which need really close analysis. I think if there needs to be a syntax update it should happen in a subsequent proposal.

Please do tell us where exactly this syntactic change is being suggested or implied in the Proposed Solution section, and we will gladly remove it..