Making map2Conserve occupy constant stack space in spirit of SI-2411.

I recently discovered a StackOverflowError, caused by this function: https://gist.github.com/folone/7b2f2e2a16314ab28109
The circumstances are pretty extreme, still having a tail-recursive function seems to be a good idea.
The function behaves the same way old function did (supported by tests), which is not really how map2 behaves. I did not change this behavior to not introduce any regression. I actually tried to make it behave like map2, and it does introduce regression.
I also think, I found a better home for this function.