[swift-evolution] Draft Proposal: count property for enum types

[Brent Royal-Gordon]

> I propose adding a read-only ‘count’ property to enum types, that returns the number of cases; modelled on the API for arrays types.

Most discussions of this start with a `count` proposal, proceed to an `allValues` array or something of the sort, and then fizzle out before anyone actually writes a formal proposal. Feel free to break that pattern.

-- 
Brent Royal-Gordon
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