Use `diff` crate while evaluating `compile-fail` tests

[cengiz-io]

Hello,

Currently we're using some internal functionality for comparing compile-fail results.

We should be using diff instead.

Thanks @nikomatsakis for suggesting this.

I'm working on this now.

[nikomatsakis]

👍

[cengiz-io]

Previously, we were dumping the differences between expected and actual compile-fail output like this

  0 - |E0282: type annotations needed|
    + |E0282: testing testing testing|

Now, it's something like this.

-E0282: type annotations needed
+E0282: testing testing testing

So, output-wise, no breaking/confusing changes introduced by this implementation change.