Upgrade 'which' to remove indirect dependency on 'failure'.

[prokopyl]

While using bindgen I noticed it brought along the failure crate, but only through the which dependency, and bindgen not even using it itself.

In an effort to remove the dependencies I don't need in my project, I therefore submitted a PR to which (harryfei/which-rs#14) to enable dependent crates to not bring in failure and its dependencies, which is what this PR is about! 😃

This PR upgrades which to version 3.0 and disables its default features, effectively removing the failure dependency from bindgen as well as its sub-dependencies: backtrace, backtrace-sys, and rustc-demangle. Since bingen uses none of them, this is only a dependency change, there is no code change in bindgen itself.

I also noticed #1615 a little too late, and while the issue is similar I believe those two PRs are actually orthogonal: #1615 allows to remove which for the users who don't need to find rustfmt (or use it altogether, which is actually my use-case in the end 🤷‍♂️ ), while this PR makes sure to remove failure and its dependencies across the board, regardless of the user's usage of rustfmt.

[emilio]

Thanks, this looks great!

[bors-servo]

☔ The latest upstream changes (presumably 43f8968) made this pull request unmergeable. Please resolve the merge conflicts.

[emilio]

Pushed the rebased version of this in 8fd16b9. Thanks!