supplant the old interface, which relied on the Num typeclass.
MixedAmount did not have a very good Num instance. The only functions
which were defined were fromInteger, (+), and negate. Furthermore, it
was not law-abiding, as 0 + a /= a in general. Replacements for used
functions are:
0 -> nullmixedamt / mempty
(+) -> maPlus / (<>)
(-) -> maMinus
negate -> maNegate
sum -> maSum
sumStrict -> maSum
Also creates some new constructors for MixedAmount:
mixedAmount :: Amount -> MixedAmount
maAddAmount :: MixedAmount -> Amount -> MixedAmount
maAddAmounts :: MixedAmount -> [Amount] -> MixedAmount
Add Semigroup and Monoid instances for MixedAmount.
Ideally we would remove the Num instance entirely.
The only change needed have nullmixedamt/mempty substitute for
0 without problems was to not squash prices in
mixedAmount(Looks|Is)Zero. This is correct behaviour in any case.
