Refactor loopUnrolling autotuner

[karoliineh]

Refactor some code in loopUnrolling.ml, simplifies the following:

• Use Log instead of printing to stderr
• Improve debug messages and simplify their syntax
• Make sure the goal (bound) of the loop is found from the exact same loop statement comparison expression where the var was found
• Move functions defined in fixedLoopSize to top level to make function definitions' style consistent
• Use let* option monad syntax from GobOption.Syntax in LoopUnrolling

[sim642]

The conflict with master now needs resolving too.