[swift-evolution] [Review] Replace `typealias` keyword with `associatedtype` for associated type declarations

[Brent Royal-Gordon]

> On a re-read I am -1; I like the `associatedtype` keyword but didn’t realize there was no plan to let `typealias` be used within a protocol to as a convenience (and to preserve intent, and to improve the development experience when still figuring out an interface design).
> 
> I would prefer the new keyword and also adding/allowing one to add convenience typealiases within a protocol definition.

I would love to see convenience typealiases supported in protocols eventually, but I actually think that should wait until after the transition to `associatedtype` is well behind us so we can get people used to the new keyword.

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Brent Royal-Gordon
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