libstdc++/9828: Regression: Segmentation fault in num_put::put

[Benjamin Kosnik]

>Yep. This is what I was talking about when this first went in, abit very
>vaguely. If you could do something like has_cache/use_cache as analogues
>to has_facet/has_cache I think we'll be better off.

Ugh. I mean: has_cache/use_cache like has_facet/use_facet! I didn't get
this right the first time I posted it.

Also, this will make the zero-allocation bits easier to do. (We need a
way to do zero allocation for both the standard streams, and for the
future, when persistent streams are done.)

best,
benjamin