Exclude unneeded files from the crate package.

[RazrFalcon]

This decreases *.crate file size from 282KiB to 90KiB.

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This change is 

[SimonSapin]

I personally don’t care much, but some people have files bugs on some other crates when cargo test doesn’t work in the result of extracting a .crate file from crates.io. I suppose it’d be nice to have a community-wide convention of what’s appropriate to do/expect around this.

@nox, do you have an opinion?

[bors-servo]

☔ The latest upstream changes (presumably #517) made this pull request unmergeable. Please resolve the merge conflicts.