This was sed-converted from the status quo. We can redefine \indexhdr to something else at a later time.

This will need some more changes once we land the motions (we're adding two new headers), but this looks good, thanks!

I've rebased this, fixed mentions of the additional headers, and while doing so, fixed bad index entries <<syncstream>> and <<type_traits>>. (\idxhdr implicitly adds the <>.)