Allow for (or require) named tuples as children

Functors.jl now returns named tuples as children. It would be nice if we could have the same behavior also here for the default walk style and supported or required for custom walkstyles

Start by reviewing the default walk style and the custom walkstyles, then compare how Functors.jl returns named tuples as children. Define whether named tuples should be supported or required in both cases, and verify that the chosen behavior works consistently for default and custom walkstyles.

Written by the indexing model from the issue text.