Use prettify-symbols-mode

[gracjan]

Use prettify-symbols-mode instead of hand crafted unicode composition.

[gracjan]

Can't use this due to the following reasons:

• prettify-symbols-mode works well on identifiers (\\_ and \\w) only as it puts \\_< and \\_> kind of boundary
• the above renders it pretty useless for operators that have punctuation character class
• it can compose part of operator only
• dot is heavily overloaded in Haskell as it can be: 1) composition, 2) part of other operator, 3) module selector, 4) dot after forall keyword

So we need to keep using our handcrafted version. That is ok as it opens possibilities for improvements.