This makes it much easier to update the bounds.  Instead of coming up
with a suitable (min,max) pair, you just give e.g. (base, 10) to allow
10% deviation from the base figure, which can be pasted from the
error.

e.g. previously:

-      # expected value: 458700632 (amd64/Linux):
-      if_wordsize(64,
-          compiler_stats_num_field('bytes allocated', 440000000,
-                                                      480000000)),

now:

+      if_wordsize(64, # sample from amd64/Linux 15/2/2012
+          compiler_stats_range_field('bytes allocated', 360243576, 10)),

Note: use stats_range_field rather than stats_num_field.  I left
support for the old way for now so that we can do a gradual migration.

(next I suppose we should make it so that 'make accept' works for perf
tests, but that's for another day)