haddock: Tidy up the codebase's pragmata, extensions, ghc flags

This MR enables some extensions and GHC flags in the cabal file in a way that allows us to reduce the amount of prologuing on top of each file. No behaviour changes are done.

We also prefix the usage of some List functions that removes ambiguity when they are also exported from the Prelude, like foldl'. In general, this has the effect of pointing out more explicitly that a linked list is used.

Moreover, some -O2 were appearing in the cabal file, but it is bad form to do so, as this belongs in a release process rather than unconditionally during the development process.

haddock-library gets no extensions by default because its support window is not clarified yet. See #24990 for the discussion