Add some more arithmetic, logic, set, type, monoid and monad notation

Except for 'return' and 'join', all of them already used in

  http://hackage.haskell.org/package/base-unicode-symbols

Eta and Mu are common notations for 'return' and 'join', e.g.

  http://www.euclideanspace.com/maths/discrete/category/higher/monad/index.htm

Even though they are letter-functions (similar to as we use lambda symbol
for '\'), they can be easily made distinguishable from letter-variables by
the colorscheme (using colors or bold type, alike to vectors vs. variables
in math.)
Add some less common notation

These are not strictly mathematical/agreed upon notation symbols. But they
do the job of making the code less cluttered with the multi-symbols names.

To enable this notation one has to place this line into ~/.vimrc:

  let informal_haskell_conceal=1
Add (arguably) better, single-space notation for monads

Vim has some issues concealing with composite symbols like '«̳', and
unfortunately there is no other common short notation for bind. But for
readability we still want a single-space character. For the time being,
these circled letters/mnemonics might work:

 b - normal bind, to the right (b "looks" to the right)
 d - bind to the left (mirrored b, "looks" to the left)