haddock: Remove unused pragmata, qualify usages of Data.List functions, add...

haddock: Remove unused pragmata, qualify usages of Data.List functions, add more sanity checking flags by default

This commit 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.

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.

Metric Increase:
    haddock.Cabal
    haddock.base
    haddock.compiler