cosmetic changes in ‘haskel-indentation.el’

Various cosmetic corrections. All the changes do not introduce any
differences in functioning of the module, this is just refactoring.

Added TODO items:

* TODO eliminate magic number 2 where possible, use a variable

* TODO `haskell-indentation-find-indentation' — fix it, get rid of "safe"
  version

I've written doc-strings where possible. Many functions still need
decent doc-strings, I've marked them with FIXME. It's possible that I'll
add doc-string of these functions later. N.B. the module is quite hairy,
so you should have really good doc-strings here.

Author: mrkkrp