Updating to serde-yaml 0.9

[plugwash]

Hi, I am currently trying to update the package of the serde-yaml crate in Debian. to 0.9.x I tried bumping the dependency in the treediff crate but the build fails with

   error[E0004]: non-exhaustive patterns: `serde_yaml::Value::Tagged(_)` not covered
      --> src/value/serde_yaml.rs:14:15
       |
    14 |         match *self {
       |               ^^^^^ pattern `serde_yaml::Value::Tagged(_)` not covered
       |

I would appreciate it if you can take a look and determine an appropriate way to handle the new enum variant.