"Default" mappings. The set of mappings to use as "default" is specified
by the AP. Each entry is keyed by the owning mixin, with the value map
containing the actual remappings for each owner
data
java.util.Map<K,V> data
All mapping sets, keyed by environment type, eg. "notch", "searge". The
format of each map within this map is the same as for ReferenceMapper.mappings