alternative to xa_for_each?

[Christoph Hellwig]

Hi Matthew,

I'm just reviewing a patch adding an xarray user, and I'm wondering
if we could just replace xa_for_each with a loop on
xa_find_after with a magic index to start from the beginning?  That
would always seem like more readable code than the magic looping macro.

And while we're at it:  is there an idiomatic way to get the entry with
the highest index?