[#185] Enable more warnings and fix them

[gromakovsky]

Description

Here I enable all warnings that I find sane, enable -Werror and fix new warnings.

Related issues(s)

Fixed #185.

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[int-index]

It's better to enable -Werror on CI only instead of always, otherwise development and building on newer GHC versions with allow-newer: True becomes difficult.

[gromakovsky]

I can add a Makefile which would pass --ghc-options -Wwarn. In this case if we switch to another CI we won't accidentally forget to enable -Werror.

[int-index]

Imagine there's a package x which depends on universum. How would it disable -Werror? Passing -Wwarn would only affect x itself, and having a Makefile for universum would not be helpful at all.

[gromakovsky]

Does --ghc-options -Werror in universum.cabal matter when one builds x? I thought no, but if it matters then yes, probably it's better to do it differently.

[int-index]

Does --ghc-options -Werror in universum.cabal matter when one builds x?

Yes. ghc-options from the .cabal file are in effect if the package is being built as a dependency of another package (why would they be ignored?)

[gromakovsky]

Ok, now it's different.