all: add algebraic enum types

[RestartFU]

Add algebraic enum types with payload variants, exhaustive switches,
generic support, and compiler, runtime, and standard tooling support.

Design: golang/proposal#60

For example:

type Result[T any] enum {
	Ok { Value T }
	Err { Err error }
}

func printResult(result Result[int]) {
	switch result {
	case Ok:
		fmt.Println(result.Value)
	case Err:
		fmt.Println(result.Err)
	case nil:
		fmt.Println("unset")
	}
}

func example() {
	result := Result[int].Ok{Value: 42}
	printResult(result)
	fmt.Println(result.Variant()) // "Ok"
}

Go+ (https://github.com/goonward/goplus) is an installable experimental
fork implementing the change end to end. It provides downloadable
toolchains and editor integrations, and serves as the working proof of
concept for this proposal.

Updates #19412

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[RestartFU]

@googlebot rescan

[HashimTheArab]

Holy shit

[gopherbot]

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/802880.
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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

Congratulations on opening your first change. Thank you for your contribution!

Next steps:
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[gopherbot]

Message from Sean Liao:

Patch Set 1: Hold+1

(1 comment)

---

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After addressing review feedback, remember to publish your drafts!

[gopherbot]

This PR (HEAD: ab9f7e5) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/802880.

Important tips:

• Don't comment on this PR. All discussion takes place in Gerrit.
• You need a Gmail or other Google account to log in to Gerrit.
• To change your code in response to feedback:
  • Push a new commit to the branch used by your GitHub PR.
  • A new "patch set" will then appear in Gerrit.
  • Respond to each comment by marking as Done in Gerrit if implemented as suggested. You can alternatively write a reply.
  • Critical: you must click the blue Reply button near the top to publish your Gerrit responses.
  • Multiple commits in the PR will be squashed by GerritBot.
• The title and description of the GitHub PR are used to construct the final commit message.
  • Edit these as needed via the GitHub web interface (not via Gerrit or git).
  • You should word wrap the PR description at ~76 characters unless you need longer lines (e.g., for tables or URLs).
• See the Sending a change via GitHub and Reviews sections of the Contribution Guide as well as the FAQ for details.

[gopherbot]

Message from Sean Liao:

Patch Set 2: Hold+1

---

Please don’t reply on this GitHub thread. Visit golang.org/cl/802880.
After addressing review feedback, remember to publish your drafts!