Move map2Conserve to a new home, next to map2.
Make 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.
DefDef.name is now TermName again

Now when there's no hope left for type macros, it's reasonable to provide
a more specific type for DefDef.name.
Merge pull request #2812 from xeno-by/topic/defdef-name-now-termname

DefDef.name is now TermName again
Merge pull request #2764 from folone/master

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