sl/join_status: a separate .hh file for the enum definition
sl/symtrace: use JoinNode also for isomorphism and entailment
sl/symtrace: get ready to track id mapping of join nodes
sl/symtrace: support join nodes in resolveIdMapping()
sl/symjoin: export ID mapping from joinSymHeaps() [not tested]
sl/symstate: pick join trace nodes also for isomorphism