Propose `enum` be a subtype of `flags`.

[sunfishcode]

I was exploring some hypothetical use cases where it'd be nice to migrate from an enum, "pick one of these", to a flags, "pick any number of these".

For example given these types:

enum berry-enum {
   blueberry,
   strawberry,
   raspberry,
}

flags berry-flags {
   blueberry,
   strawberry,
   raspberry,
}

The proposal is berry-enum <: berry-flags.

I know that the subtyping rules aren't currently a focus, and that they may change when the time does come to focus on them, but I wanted to add this idea to the list so that it's considered.

[lukewagner]

That's an interesting idea, let's tag it as an enhancement and consider it when we're defining/implementing value-level subtyping properly. As #229 suggests, in a preview2 timeframe, we're starting without any value-level subtyping, so probably we'll want to consider in detail the forms of subtyping we do want to allow as a post-preview2 batch.

[lukewagner]

I'll close for now, since subtyping is generally removed from the C-M repo (#255), but we can revisit when we properly treat subtyping in the future. (Feel free to file a new issue to document the idea that we can tag appropriately.)