Get rid of build warnings related to monoids

[turion]

When building with GHC 8.8, There are a lot of warnings like this:

The import of ‘Data.Semigroup’ is redundant

These places should be refactored to make build less noisy.

[Temurson]

Hello! I would love to fix this. Currently, I have cloned the repo and managed to build the project. I got the same warnings. I will try to make a PR soon (if I figure out how to do this).
Should I only fix warnings about Data.Semigroup? Only -Wunused-imports? All of them?

[turion]

@Temurson thanks for your enthusiasm, but this issue was already fixed in #192. There are lots of other open issues, though ;)

[turion]

If you need any help making a PR, I'll gladly assist!