Sort out completing-read

Completing read is a must-have these day. `haskell-mode` has some mechanisms to provide it, those need a clean up.

``` el
grep complet.*read *.el
haskell-cabal.el:;; (let ((section (completing-read
haskell-complete-module.el:(defun haskell-complete-module-read (prompt candidates)
haskell-customize.el:(defcustom haskell-completing-read-function 'ido-completing-read
haskell-customize.el: (function-item :tag "ido" :value ido-completing-read)
haskell-customize.el: (function-item :tag "helm" :value helm--completing-read-default)
haskell-customize.el: (function-item :tag "completing-read" :value completing-read)
haskell-load.el: (haskell-complete-module-read "Module: " modules)))
haskell-modules.el: (haskell-complete-module-read
haskell-session.el: (let* ((session-name (funcall haskell-completing-read-function
haskell.el: (let ((module (haskell-complete-module-read
haskell.el: (funcall haskell-completing-read-function "Cabal command: "
```

These is some special code `haskell-complete-module-read`, what does it do?

To do:
- [ ] use `funcall haskell-completing-read-function` everywhere
- [ ] remove `haskell-complete-module-read` (Note: need to check what this really is)
- [ ] find out what is going on in `haskell-complete-module.el`. Either write proper documentation and test cases for it or remove the thing.