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Can't find much about this in Arrow's PR history, but perhaps I didn't look thoroughly enough. What was the reasoning behind deprecating Semigroup, Monoid, etc? I'm thinking of experimenting with them in `arrow-context`, but if there's strong rational for why they'd be bad, then it's probably not worth wasting time on it.

there are several intertwined reasons, but some of them are:
• it deviated from Kotlin idioms: for example in `fold` your take empty value and combine function, not a single class containing both
• since Monoid has two methods, it's not possible to use Kotlin's lambda-conversion feature, so the code looked even harder to understand
• you had to pass the Monoid explicitly, even if you were using it in several place

it may be that the third problem is no longer present with context parameters, but I wouldn't put that much work on it either

Hmmmm. Semigroup could be interesting because it doesn't have those issues, but it's also really insignificant because passing a `combine` function really isn't a big issue at all.
I think I'll pass on it for now :)