A few additions to vim-haskellConceal

Hello,

There were a few changes that I had in my local tree for some time. Recently I've got some time to cleanup and split them for review/pull request.

The changes are logically separated, please see the commit messages for the justification. I guess the main addition is in the following patch:
  'Add some more arithmetic, logic, set, type, monoid and monad notation'
The notation there was taken from the base-unicode-symbols package, so it should be pretty universal.
The last two patches in the series are using informal notation (protected by a flag). If you don't like any of the commits, I can remove them from the for-upstream branch and resend.

Thanks!