errors: add new function Temporary(error) bool

[aka-rider]

This is proposal to add the Temporary function into errors package. I tend to rewrite it over and over again in every project.

Mainly, this function will be used in conjunction with the net package.
Nevertheless, I think Temporary function belongs to the erros package, in this case user code and other packages may follow the same convention.

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Patch Set 1: Code-Review-2

New API like this should normally go through the proposal process. See https://golang.org/s/proposal.

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[aka-rider]

For tracking #37250

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Patch Set 1:

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

Next steps:
Within the next week or so, a maintainer will review your change and provide
feedback. See https://golang.org/doc/contribute.html#review for more info and
tips to get your patch through code review.

Most changes in the Go project go through a few rounds of revision. This can be
surprising to people new to the project. The careful, iterative review process
is our way of helping mentor contributors and ensuring that their contributions
have a lasting impact.

During May-July and Nov-Jan the Go project is in a code freeze, during which
little code gets reviewed or merged. If a reviewer responds with a comment like
R=go1.11, it means that this CL will be reviewed as part of the next development
cycle. See https://golang.org/s/release for more details.

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Please don’t reply on this GitHub thread. Visit golang.org/cl/219497.
After addressing review feedback, remember to publish your drafts!