Flatten 'extraConceal' if-block

The patch looks scary but it is actually trivial: instead of having the
huge indented if-block, we can just have

  if !exists('s:extraConceal')
      finish
  endif

at the beginning.

This patch makes it easier to add other flags.
Introduce 'tau' constant and '<<' operator

The circle constant (tau) slowly becomes quite common (it is equal to
2*pi), popularized in Haskell community through the Diagrams package:

  http://projects.haskell.org/diagrams/haddock/Diagrams-Util.html#v:tau

'<<' is commonly used in Text.Html.
Use 'not identical to' unicode symbol for '/='

We already use 'identical to' for '==', so let's be consistent and use
'not identical to' for '/='. The base-unicode-symbols[1] package uses both
'not equal' and 'not identical'.

[1] http://hackage.haskell.org/package/base-unicode-symbols
Use 'times' sign for multiplication

Base-unicode-symbols[1] uses dot for multiplication, but most of the time
it's indistinguishable. Let's use 'times' symbol. If anything, it is
better than a star.

[1] http://hackage.haskell.org/package/base-unicode-symbols
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.
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)