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).