multi-types may work on child indexes

[madnificent]

The key composite_types allows to combine types of content in one response. The readme names this "Multi-types".

These types can be calculated at indexing time, or we could combine the indexes at query time. The latter approach seems easier to implement. To our knowledge, multi-types are currently not used in the wild.

In the light of a cleanup operation in some code segments, we will partially remove the multi-types and we will attempt to add them again using the at-query-time integration. This change as per suggestion of @nvdk

Please close this ticket once multi-types using the composite_types key are supported again.