Pull in ghc-mod's ghc-make-indent-shallower/deeper into haskell-mode

[gracjan]

Do:

• pull code from https://github.com/kazu-yamamoto/ghc-mod/blob/master/elisp/ghc-indent.el
• connect to haskell-mode-map under keys C-c < and C-c >
• use haskell-indentation-layout-offset for calculations
• create a pull request against ghc-mod that removes this functionality from there
• describe new functionality in doc/haskell-mode.texi

Mentioning @kazu-yamamoto, so that he knows what we are up to.

[kazu-yamamoto]

Assign this job to me. I will send a pull request.

[gracjan]

@kazu-yamamoto: Actually I'd like to spare this task for somebody that is a less experienced lisp hacker. What you can do though is to try to find and describe more bite-sized tasks so that other people can jump in and help easily.

[kazu-yamamoto]

OK.

[gracjan]

@kazu-yamamoto: I see that nobody volunteered. Can you do this one yourself?

[gracjan]

Interestingly haskell-indentation-mode does indent-rigidly when pressed TAB or S-TAB while a region is active. Is this the same functionality or does ghc-mod add functionality on top of that?

[gracjan]

Closing in favor of #949.