Add `partition: true` option to `belongs_to`

Allows defining all of the parent association keys as partition keys for
the child class.
Specs for `belongs_to` with `partition: true`
Line length
Merge branch 'belongs_to_partition'

* belongs_to_partition:
  Line length
  Specs for `belongs_to` with `partition: true`
  Add `partition: true` option to `belongs_to`