SList: correct reference semantics
SList: use NodeWithoutPayload for root node
SList.Node: remove constructor
Fix reduce regression 13304.

Also cover test case 10709.
Merge pull request #2401 from sigod/slist-correct-reference-semantics

SList: correct reference semantics
Merge pull request #2431 from monarchdodra/13304

Fix reduce regression 13304.