As promised in #25462 comment:56, this provides a fast iterator for set partitions with given block sizes, via linear extensions of a certain poset.

Depends on #25462
Depends on #25864

CC: @tscrim @kevindilks

Component: combinatorics

Author: Martin Rubey

Branch/Commit: 027b299

Reviewer: Travis Scrimshaw

Issue created by migration from https://trac.sagemath.org/ticket/25865