The idea is to avoid std::map lookup for each CellRef. Instead generate a
sequence of added and removed RefNums into a vector then order them by RefNum
using a stable sort that preserves relative order of elements with the same
RefNum. RefIDs are stored in a different vector to avoid std::string move ctor
calls when swapping elements while sorting. Reversed iteration over added and
removed RefNums for each unique RefNum is an equivalent to what map-based
algorithm produces. The main benefit from sorting a vector is a data locality
that means less cache misses for each access. Reduces ESMStore::countRecords
perf cycles by 25%.
