- Consume list immediately in commodityStylesFromAmounts.

  This and/or the other strictness fixes below reduce memory and time
  usage, a lot, for journals with many @@ total prices.

- Include sign in TotalPrice in Amount, rather than relying on the sign of aquantity.
  Journal entries still require a positive @@ price, but now the sign is
  set after parsing, rather than when converting in amountToCost.
  The reason for this change is that, if we're going to perform arithmetic
  on Amount with TotalCost, then the presence of aquantity=0 means that
  amountToCost would render the total cost as 0, because signum 0 == 0.

  This makes amount arithmetic more consistent, and allows negative
  prices to be represented. It also means that total prices override
  the primary amount: 0A @@ 1B is now 1B, not 0.

- Make fields of Amount, AmountPrice, AmountStyle, and DigitGroupStyle
  strict.

- Distinguish between an Amount having quantity (or rounded quantity 0),
  and having both quantity and totalprice 0 (or rounded to 0).

- normaliseHelper now uses a strict Map for combining amounts
  internally, closing a big space leak.

  This also now combines Amounts with TotalPrices in the same commodity
  when normalising; amounts with TotalPrices were previously never
  combined.

- (amount|mixedAmount)(Looks|Is)Zero functions now check whether both
  the quantity and the cost are zero. This is usually what you want, but
  if you do only want to check whether the quantity is zero, you can run
  mixedAmountStripPrices (or similar) before this.