Return data from heap_page_prune via a struct.

Return data from heap_page_prune via a struct.

Previously, one of the values in the struct was returned as the return
value, and another was returned via an output parameter. In
preparation for returning more stuff, consolidate both values into a
struct returned via an output parameter.

Melanie Plageman, reviewed by Andres Freund and by me.

Discussion: https://postgr.es/m/CAAKRu_br124qsGJieuYA0nGjywEukhK1dKBfRdby_4yY3E9SXA%40mail.gmail.com