zero-alloc cache (was Re: [v3] Fix PR libstdc++/10276)

[Benjamin Kosnik]

[problem with adding iword/pword to standard streams not getting deleted]

>The problem I indicated above still exists, though.  Is it a defect in
>the standard?  Even if not, perhaps we should add it as a caveat to
>the (nonexistent) pword docs?

I don't know if it's a defect, but it might bite people. Let's add it on
page 2 of the imaginary docs!

-benjamin