sl/symabstract: store ID mapping to Trace::AbstractionNode
sl/symabstract: store ID mapping to Trace::ConcretizationNode
sl/symabstract: minor cleanup in dlSegReplaceByConcrete()
sl/fixed_point_proxy: label cont-shapes trace edges

... by the count of shape-to-shape edges they actually represent
(in case the count is greater than one)