Implements iterator data/operations:

• HeapScanDesc ... struct containing iteration state
• scan = heap_beginscan(rel,...,nkeys,keys)
  (uses initscan() to do half the work (shared with rescan))
• tup = heap_getnext(scan, direction)
  (uses heapgettup() to do most of the work)
• heap_endscan(scan) ... frees up scan struct
• HeapKeyTest() ... implements key match test