Refactor and remove YAML_mapping

[crimson-knight]

In the Amber shard file, we still rely on yaml_mapping which is a shard created to maintain the deprecated YAML.mapping functionality that was moved into YAML::Serializable in the std lib.

This is currently in the shard.yml file and can be found inside of the CLI tool and Amber-core.

Let's remove this deprecated library and reduce unnecessary shards from being installed with Amber.