It now uses the same JSON representation as Maybe Word8. This means that
the JSON serialisation is now broadly compatible with that used before the
commit f6fa76bba7, differing only in
how it handles numbers outside Word8 and that it can now produce null
for NaturalPrecision.
Amounts in JSON are now rendered as simple Numbers with up to 10
decimal places, instead of Decimal objects which would in some cases
have 255 digits, too many for most JSON parsers to handle.
A provisional fix, see the comment in Json.hs for more detail.
