This change provides more predictable and intuitive behaviour when
using -S/--sort-amount with multiple commodities.
It implements a custom Ord (and Eq) instance for MixedAmount
which substitutes zero for any missing commodities.
As a consequence, all the ways of representing zero with a MixedAmount ([],
[A 0], [A 0, B 0, ...]) are now Eq-ual (==), whereas before they were
not. We have not been able to find anything broken by this change.
* imp: lib: Compare MixedAmounts by substituting zero for any missing commodities. (#1563)
* ;doc: Update docs for new multicommodity sort by amount rules.
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