Remove __len__ from aggregate types

Having a `__len__` can produce subtle errors, since a zero-length object is considered false-ish by Python. Aggregate types already have a `elements` attribute which can be used instead.

Start by locating the aggregate type definitions and their existing __len__ implementations; the issue does not name files or tests. Check how the elements attribute is exposed, remove __len__ from each aggregate type, and verify that aggregate objects no longer provide length-based truthiness while elements remains available.

Written by the indexing model from the issue text.