Indirect formatted scanning through the scanstate.

This will allow us to reuse the scanning protocol with an arbitrary area
scanning function (replacing traceFormatScan) in order to implement
formatted object walking without an extra segment method.

Don't insist on scanning only grey segments: we want to be able to
reuse the scan protocol for walking, when the segments are black.

mps/code/mpmst.h

@@ -402,6 +402,8 @@ typedef struct ScanStateStruct {
   Sig sig;                      /* <design/sig> */
   struct mps_ss_s ss_s;         /* .ss <http://bash.org/?400459> */
   Arena arena;                  /* owning arena */
+  mps_area_scan_t formatScan;   /* formatted area scanning function */
+  void *formatScanClosure;      /* closure argument for .formatScan */
   SegFixMethod fix;             /* third stage fix function */
   void *fixClosure;             /* see .ss.fix-closure */
   TraceSet traces;              /* traces to scan for */