D-YAML v0.9.0 released (DIP1000 compatibility)

[Mathias LANG]

Hi everyone,
A few days ago, I released 
[v0.9.0](https://github.com/dlang-community/D-YAML/releases/tag/v0.9.0) of D-YAML.
D-YAML is the most used package for parsing YAML file, and part 
of dlang-community.
This new release makes the library compatible with DIP1000, which 
required getting rid of Variant (but forking `std.sumtype`).