Verify documentation links on travis

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Fixes #82

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@stjepang when you have a moment would you please take a look at this and let me know if it's what you had in mind for resolving #82? I think this should fix all the current errors produced by cargo deadlinks --check-http.

If it looks okay, I'll squash the commits and mark it ready for review.

One thing that will probably need to be changed is the installation step for cargo-deadlinks. Should I just create a ci/install-cargo-deadlinks.sh and follow the same template as ci/install-mdbook.sh?

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Let's also delete this section.

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If I delete these doc shims in src/os/unix/fs.rs, then I start to get errors like this:

Found invalid urls in async-std/target/doc/async_std/os/unix/fs/index.html:
        Linked file at path async-std/target/doc/std/fs/struct.OpenOptions.html does not exist!

I tried to trace back to where all of these are being referenced but wasn't successful. Do you still want to try and remove them?

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One thing that will probably need to be changed is the installation step for cargo-deadlinks. Should I just create a ci/install-cargo-deadlinks.sh and follow the same template as ci/install-mdbook.sh?

Yes, that sounds reasonable to me!

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@stjepang Thanks for the feedback!

Responding to a few specific points you raised:

1. I've been pinning the toolchain since I work on Windows sometimes and the build is (was?) broken on current nightly.

2. The trait@Send links do indeed work and they are currently pointed to the docs upstream.

3. I've been unable to remove some of the shims in os/unix/fs.rs; see comments above.

Finally, installing cargo-deadlinks following the same template as ci/install-mdbook.sh isn't going to work. The reason is because the script at https://japaric.github.io/trust/install.sh has the release download url hard-coded for a naming format the deadlinks/cargo-deadlinks repo doesn't use.

Given that, I'm not sure how you want to handle the installation. Thoughts? (cc @jamesmunns)

Otherwise, I've made the requested changes.

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Going to close this. If someone would like to resume the work, feel free to ping me.