[Breaking change] Restructure terminology to reflect differences
between series and multi-volume works

Rename 'part_of' → 'series', 'host_item' → 'part_of', 'series' →
'is_series', and add 'is_multivolume'.

The inverse relationships are now

Host:              Part:
'is_multivolume'   'part_of'
'is_series'        'series'

I'm still not sure if the differences between multi-volume works and
series justify different terminology, with the cons that follow with
increased complexity. An alternative would be to use 'has_parts' and
'part_of' for both. But then, from 'has_parts' we would expect actual
links to the parts, which we don't have in general.