rustfix should handle multi-span suggestions

[zackmdavis]

Here:

https://github.com/rust-lang-nursery/rustfix/blob/89217428c130b62c7ddaa0734eb4ebaa33673a86/src/lib.rs#L191-L198

This issue concerns the matter of a single suggestion that happens to replace multiple spans (functionality that is not used often, but which does exist). This is not to be confused with multiple suggestions that happen to touch the same code, which (if I understand correctly while reading quickly) was what #140 was about.

I'm happy to work on this because I need it for rust-lang/rust#53013.

[estebank]

CC rust-lang/rust#56220

[pietroalbini]

Fixed by #155

[ehuss]

I think in a sense this is resolved by #195. There is still an outstanding consideration that there is no way to distinguish between a diagnostic that has multiple spans, and one that has multiple variants. I think that is tracked in rust-lang/rust#53934.