Consider using a snapshot format that supports multiline strings

[jacobgardner]

JSON ends up turning line breaks into \ns which makes multiline snapshots very hard to diff for code reviews/pull requests or if you're using your diff tool instead of cargo snapshot.

I'm not a particularly big fan of YAML, but it does support multiline strings and there is a serde crate for yaml serialization.

[jacobgardner]

Doesn't look like the yaml-rust crate currently supports multiline strings, but the hjson crate does.

Requires a downgrade of serde, but since serialization to snapshots is all we're using serde for right now, shouldn't be a big deal. I'll make a PR for your consideration.