design/19412: add algebraic enum types

[RestartFU]

Add a design document for algebraic enum types.

The working proof of concept is Go+ (https://github.com/goonward/goplus), with a corresponding implementation in golang/go#80467.

Updates golang/go#19412.

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Message from Jonathan Amsterdam:

Patch Set 1:

(11 comments)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/802920.
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Message from Danick Lachapelle:

Patch Set 1:

(8 comments)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/802920.
After addressing review feedback, remember to publish your drafts!

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Message from Jonathan Amsterdam:

Patch Set 1:

(2 comments)

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After addressing review feedback, remember to publish your drafts!

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Message from Jonathan Amsterdam:

Patch Set 1:

(1 comment)

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Please don’t reply on this GitHub thread. Visit golang.org/cl/802920.
After addressing review feedback, remember to publish your drafts!