mps br/vmem: simple-chunk-return:

zcoll.c:
 How to get rid of all the objects, so full collect really collects all automatic objects:
   - Rootdrop() helps, but we can still retain a 1.2MB object;
   - stackwipe() does not help much -- these unwanted ambig refs are being left on the stack by MPS code that runs between mps_arena_collect and the flip!
   - therefore StackScan(0/1) to destroy stack+reg root before full collect: it's the only way to be sure.

 Reproducibility:
   - give Make() a random? switch, acted on by df() = diversity function, to allow bypass of rnd();
   - ZRndStateSet, to set the seed for rnd()

 Output:
   - print_M: switchable Mebibytes or Megabytes (more useful, to be honest);
   - get(): don't report message times, it messes up diffs.

 testlib.c/h:
 Reproducibility:
   - fix rnd_state so a rnd_state getter is possible;
   - testlib.h += rnd_state_t, rnd_state(), rnd_state_set(), rnd_state_set_v2()

 trace.c: traceFindGrey diag: no newline please

Copied from Perforce
 Change: 170093
 ServerID: perforce.ravenbrook.com

mps/code/trace.c

@@ -974,7 +974,7 @@ static void traceFindGrey_diag(Bool found, Rank rank)
     *report_lim++ = this;
     *report_lim++ = '\0';
     DIAG_SINGLEF(( "traceFindGrey",
-                   "rank sequence: $S\n",
+                   "rank sequence: $S",
                    (WriteFS)report_array,
                    NULL ));
   }