Add a different sort of stats_num_field helper function
Uses look like stats_num_field('bytes allocated', [(wordsize(32), 45648, 5), (wordsize(64), 49400, 5)]) where the first matching triple will be used. e.g. we could override the Win32 expected values with: [(platform('i386-unknown-mingw32'), 41000, 5), (wordsize(32), 45648, 5), (wordsize(64), 49400, 5)]) with other 32-bit platforms falling through to the wordsize(32) case. This makes it easier to give different values for different platforms, while being sure that all platforms are covered.