[PATCH][libstdc++-v3 parallel mode] Clean up loser tree and multiway_merge code

[Benjamin Kosnik]

> Please approve for mainline.
> 
> 
> This also fixes PR 35588. I could do a much easier separate fix for
> the 4.3 branch, if desired.

Please commit to both gcc and gcc-4_3-branch. No need to repost for
additional typo fixes.

> > In general, the ChangeLog entries have to mention each individual
> > functions.
> 
> I will do that in the future. However, IMHO in this case, this 
> higher-level view is more helpful, since the whole thing is 
> restructured/rewritten. Is that ok for you?

I should probably be better about this too. It is easier to grep the
ChangeLogs if you have more info. That said, this is probably sufficient
for now.

-benjamin