print now shows zero posting amounts with their original commodity
symbol and the corresponding style (instead of stripping the symbol).

If an inferred amount has multiple zeroes in different commodities,
a posting is displayed for each of these.

Possible breaking changes:

showMixedAmountLinesB, showAmountB, showAmountPrice now preserve
commodityful zeroes when rendering. This is intended to improve print output,
but it seems possible it might also affect balance and register reports,
though our tests show no change in those.