wip: haddock: Move haddock-bin to its own sub-directory
This MR moves the files and directories related to the haddock
binary in their own sub-directory.
This makes the topology of the project more regular and helps circumvent a cabal bug where a relative filepath for the compiler is not stable if one project is in a sub-directory and another is at the top-level.
No behavioural change is expected from this MR.