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

[B. Kosnik]

Jerry, I'm taking this as your white flag.

>In addition, there are strings in the cache.  How do these avoid
>allocating?  There is also another issue flagged by valgrind, but I
>don't know what it is from:

This is the internal string memory managment. You'll need to use an
allocator in your strings that is pre-allocated, or not use strings.

-benjamin