some code cleaning and complexity improvements

[zapashcanon]

Hi,

I was reading the code and decided to do three things:

• clean code with things should not be controversial and are clear improvements (first commit and second commit)
• fix the complexity of a few operations (the List.filteri and List.compare_length_with changes), one of them was quadratic for no good reason (second commit)
• explicit some parameters, this allows to get less false positives warnings of the kind "this argument will not be used by the function" when one still wants to enable warnings in its editor without having propoer support through the dune build system, this may be more controversial and I'm fine with removing it (third commit)

Thanks!

[zapashcanon]

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