Internally, field mappings should be stored at the index level, rather than at the type level. Types are essentially a way of grouping fields. While the mapping APIs will not change, an exception will be thrown if an attempt is made to map a field in one type in a way that conflicts with the mapping of a field with same name in another type.

The _parent field is the one exception - this field will remain at the type level.

Relates to #8870