Switch to rust-pretty-assertions for assertions

[casey]

https://github.com/colin-kiegel/rust-pretty-assertions

There have been a few times that equality assertions have failed with very slightly different text. This would help enormously in being able to see immediately how the text differs

[casey]

Actually, rust-pretty-assertions turned out to be less useful than I thought. It colored large areas of text around the non-matching characters, which made the actual errors hard to pick out.

Closing.

Also, this seems to be coming down the pipe: rust-lang/rfcs#1866