Yaml merge keys

This crate implements support for the Merge Key Language-Independent Type for YAML draft specification for YAML documents from the yaml-rust and serde_yaml (with the serde_yaml feature) crates.

When a mapping in a YAML document contains a << key, its value should be either a mapping or a sequence of mappings. For each mapping, it is merged into the parent mapping where the parent mapping wins conflicts (so that it may override keys from the merge set).