Use flycheck to manage error and warning overlays

[gracjan]

flycheck has achieved status of emacs standard for on-fligh error and warning reporting. It provides visually apealing indicators, infrastructure to handle more information, keybindings and user interface. There are plans to expand the functionality to provide fixing behavior.

There is no need to reimplement all of the functionality.

Tasks to do:

• make haskell-mode depend on flycheck
• remove current overlay functionality
• when loading a file in interpreter gather all errors and warnings in a respective session, not that it should gather all errors and warnings for all files that are loaded in a session
• when flycheck requests error list for a specific buffer provide it from the session

Use the :start hook in flycheck checker definition to achieve this.

[gracjan]

Flycheck declared error fixups as low priority flycheck/flycheck#530, it means we need to do it ourselves.

Closing in favor of #1170.