Add a publish-build-info command for reporting build results to crates.io

[carols10cents]

This goes with rust-lang/crates.io#540. @shepmaster worked on these too :)

This PR adds a cargo command, publish-build-info, which allows crate owners to report crate version, rust version, target, and pass/fail to crates.io.

The purpose is to enable crate authors to automatically report to potential crate users on the Rust version and platform compatibility of each version of the crate.

The idea is that this command would be added to CI, and an example of configuration for Travis is in the documentation we added. We haven't actually tested that configuration out yet; we wanted to get these PRs in for feedback before we set up crates.io instances that weren't local but that we could publish to :) We want to add Appveyor too, but we want to test the travis configuration first.

We also thought about, but didn't implement:

• An --overwrite flag to allow editing of an already-reported-on crate version+rust version+target combination, to allow for correcting errors if reporting manually Overwriting is now the default behavior.
• Adding flags to enable manual specification of rust version and crate version, right now cargo just detects what it's running with. Target can be overridden with --target since there was already support for that flag :)

We're excited to hear what anyone thinks, and of course happy to answer questions or make modifications!

test

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

I like it!

Trying to come up with a comment or critique, but it all seems great to me. My personal preference would be to have AppVeyor configs here as well to copy/paste before merging, but we can always add that after-the-fact too.

cc @rust-lang/tools, a nifty command to help us track version compatibility!

[bors]

☔ The latest upstream changes (presumably #3682) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #4162) made this pull request unmergeable. Please resolve the merge conflicts.

[alexcrichton]

@carols10cents was this waiting on a review? Or more coordination from crates.io?

[carols10cents]

i need to rebase the crates.io side and i think that should be merged first?

[alexcrichton]

Ah ok, just wanted to confirm!

[bors]

☔ The latest upstream changes (presumably #4680) made this pull request unmergeable. Please resolve the merge conflicts.

[carols10cents]

We're going to reopen this in a second lol

[jtgeibel]

Yeah, my bad, sorry about that! Accidentally pushed :build-info instead of HEAD:build-info and deleted the branch, confusing github.

[bors]

☔ The latest upstream changes (presumably #5907) made this pull request unmergeable. Please resolve the merge conflicts.