Add unwrap support

[vosst]

This change adds support for errors.Unwrap functionality originally introduced in Golang 1.13. While the library code does not need to take multiple Golang versions into account, the testing code needs to distinguish the cases Golang < 1.13 and Golang >= 1.13. For that, I introduced an unwrap helper with different implementation depending on the Golang version.

[palantirtech]

Thanks for your interest in palantir/stacktrace, @vosst! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[vosst]

@nmiyake I tried to sign the CLA using cla.palantir.com but only realized when looking at the DocuSign request that my primary email address on my github profile isn't my personal one. Could you help in resetting the cla.palantir.com entry for my github username such that I can sign the CLA for my personal email address? Thanks 😄

[vosst]

@dtolnay ^ because I see you on the list of contributors, too.

[asvoboda]

Hey @vosst sorry about that, can you try again now?

[vosst]

@asvoboda thanks for the swift response, signed the CLA.

[vosst]

@asvoboda can you help out with reviewing/finding reviewers here, too?

[asvoboda]

Yep, I've pinged some of our internal folks to take a look. This repo looks like it needs a bit of TLC first

[vosst]

@asvoboda happy to help out, too. Let me know if there are any tasks that I can assist with.

[vosst]

@asvoboda we switched to a project-specific fork for the time being (see https://github.com/interuss/dss/pull/426/files). Would be great to get the functionality upstream, though.