Release Note: Serial: Better GC Throughput with Scarce Dirty Cards

During a young collection, Serial GC searches for dirty cards in the card table to locate old-to-young pointers. After finding dirty cards, Serial GC uses the block offset table to locate object starts for heap-parsing to be able to walk the heap within these dirty cards object-by-object.

This change improves the object start lookup and search for dirty cards resulting in a large (~40%) reduction in Young-GC pause in some benchmarks using large object arrays.