Use monoidal arithmetic in cardano-coin-selection.

[jonathanknowles]

Issue

ADP-1449

Description

This PR makes the following replacements within the cardano-coin-selection library:

Before	After
{TokenBundle,TokenMap,TokenQuantity,Coin}.add	Semigroup.<>
{TokenBundle,TokenMap,TokenQuantity,Coin}.subtract	Reductive.</>
{TokenBundle,TokenMap,TokenQuantity,Coin}.difference	Monus.<\>
{TokenBundle,TokenMap}.empty	Monoid.mempty
{TokenQuantity,Coin}.zero	Monoid.mempty

Functions on the LHS of the above table are already just synonyms of class methods on the RHS.

This takes us one small step closer to removing the dependency on types defined within cardano-wallet-primitive.

[Anviking]

I'm not sure I'd spot the difference very easily 🤔 Could it make it unnecessarily difficult for other people as well? I guess subtract and monus / truncatedSubtraction would be clearer? Or even the unicode character ∸ wikipedia lists... Or <->? (like the ledger)

[jonathanknowles]

I'm not sure I'd spot the difference very easily

Thanks for mentioning this, I do sympathise with your argument.

I'd be very happy to discuss this together with you over a call. Perhaps we can work out a better solution.

One thing to mention is that at least when writing code, it's fairly hard to mix these up, since they do have very different types:

</> :: Reductive m => m -> m -> Maybe m
<\> :: Monus     m => m -> m ->       m

But as you point out, when reading code, it can be easy to mistake one for the other.

The way I remember <\> is that it looks visually similar to the set subtraction operator \, which is indeed the monus operation for sets.

<->? (like the ledger)

The ledger <-> operator is for group subtraction, which only works for things that are actually groups. Recall that a <-> b is equivalent to a <> invert b. But none of the types involved in this PR support a group invert operation, so <-> is not appropriate here.

or even the unicode character ∸

We could use the ∸ operator, as you mention, but that would then deviate from the standard operator used by monoid-subclasses, so people who are used to that library would have to adjust to this new operator.

Would be happy to discuss our options together over a call!

[Anviking]

lgtm modulo wanted to flag concern about ease of telling <\> and </> apart