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)