Decide on future of YAML Support (Update to serde_yaml 0.9)

[mitsuhiko]

There are now a handful of issues related to YAML in insta. #255 is a recent experiment by a contributor to drop the serde dependency which also moves to yaml-rust internally. The original issues with serde_yaml is tracked in #214.

The latest unrelated issue though is that serde_yaml in 0.9 has an incompatible serialization mode which completely changes how snapshots are rendered. This is a meta issue to figure out what to do there. For now the plan is to at the very least keep insta on 0.8 until a decision for #255 has been made.

[mitsuhiko]

I did not investigate the details yet but it very much looks like serde_yaml in 0.9 moved away from yaml-rust which in turn means that the format now is also different. It uses a C to Rust transpiled unsafe version of libyaml.

Longer term it's unclear what the maintenance of yaml-rust will be but it's not unlikely that the crate is good enough for the uses of serde. It also means that by passing serde_yaml retains the correct output format whereas upgrading to serde_yaml changes yaml snapshots (dtolnay/serde-yaml#297).

Given all this I am likely going to bypass serde_yaml for now and we cross the bridge of the future of yaml-rust when it comes to it.