Improve performance for rightOfBack and leftOfBack

[jfmengels]

I had a quick look at whether there were functions with "obvious" performance improvements, and came across rightOfBack (the List.reverse + List.head called out to me).

I tried several approaches (that you can go through in this benchmark). The final benchmark gives the following:

leftOfBack had a very similar algorithm, so I updated it as well to use the same improvements, with the following results (benchmark)

Less impressive, but still a nice improvement.

There were no tests for these 2 functions, so I added basic ones, based on the examples.