cmd/stringer: add the ability to optionally generate TypeFromString function

[bbrodriges]

This PR adds the ability to generate function opposite to String allowing to retrieve type value by its string representation.

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Message from Gerrit User 5976:

Patch Set 1:

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Message from Gerrit User 5065:

Patch Set 1:

(1 comment)

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Message from Gerrit User 12446:

Uploaded patch set 2: Commit message was updated.

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Message from Gerrit User 12446:

Uploaded patch set 3: New patch set was added with same tree, parent, and commit message as Patch Set 2.

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Message from Gerrit User 5065:

Patch Set 3:

Maybe this should lazily populate the global maps with a sync.Once to not make golang/go#26775 worse.

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Message from Gerrit User 12446:

Uploaded patch set 5: New patch set was added with same tree, parent, and commit message as Patch Set 4.

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Message from Daniel Martí:

Patch Set 5: Code-Review-2

I'm not sure why gobot pinged me now.

In any case, I'm afraid that I proposed precisely this feature half a year before you posted this PR: golang/go#23535

It got rejected, so I think that's the end for now. I'm sorry that you spent the time to work on this patch, but in the future, please remember to raise an issue before working on a new feature.

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This PR is being closed because golang.org/cl/129816 has been abandoned.

Clearing my incoming reviews queue.