use IndexedTables rather than StructArrays utils

[piever]

There already is a foreach in IndexedTables that can do the same job as foreachfield from StructArrays so it's probably safer to not rely on external "utility" functions.

[piever]

Better to wait a bit before merging here as foreachfield is actually so useful for StructArrays that I may document it / export it and make it part of the official API, so maybe this change is not needed.

EDIT: in the end I'd rather not commit to exposing foreachfield as part of the API. I'll merge here to further decouple StructArrays and IndexedTables.