Refactor haskell-navigate-imports

[gracjan]

It should work like this:

• first M-g i pushes mark using push-mark and goes to first import in file
• repeated M-g i moves to the next import block (group of import statements separated by something else)
• repeated M-g i cycles to the first one when past the last one
• standard C-u C-SPC is used to return to old point position

Half of the logic should go away.

[zzantares]

I'm also interested in getting this behavior, I have some changes that are closer to this, basically by relying on the existing implementation of haskell-navigate-imports-go-internal directly, however, jumping between import sections (as outlined in the 2nd and 3rd bullet point in the issue description) will also add a mark so jumping back visits every import section in succession (possibly cycling on reverse) before the cursor ends up at the starting point.

One approach to fix this would involve turning haskell-navigate-imports into a minor-mode that when activated remembers the initial position, then as long as this mode is active jumps between import sections won't be recorded, finally any other action but cycling imports would deactivate the minor mode that adds the intial position to the jump list right before deactivates. Maybe this is a bit contrived and not the right approach but I'm open to ideas.