First step towards rustfix compiletest mode

This is the first small step towards testing auto-fixable compiler
suggestions using compiletest. Currently, it only checks if next to a
UI test there also happens to a `*.rs.fixed` file, and then uses rustfix
(added as external crate) on the original file, and asserts that it
produces the fixed version.

To show that this works, I've included one such test. I picked this test
case at random (and because it was simple) -- It is not relevant to the
2018 edition. Indeed, in the near future, we want to be able to restrict
rustfix to edition-lints, so this test cast might go away soon.

In case you still think this is somewhat feature-complete, here's a
quick list of things currently missing that I want to add before telling
people they can use this:

- [ ] Make this an actual compiletest mode, with `test [fix] …` output
  and everything
- [ ] Assert that fixed files still compile
- [ ] Assert that fixed files produce no (or a known set of) diagnostics
  output
- [ ] Update `update-references.sh` to support rustfix
- [ ] Use a published version of rustfix (i.e.: publish a new version
  rustfix that exposes a useful API for this)

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