[RFC] Default constructed stringbufs and stringstreams

[Benjamin Kosnik]

>I like it *so* much and, if you can't spot something wrong with it, I'd
>like to commit it later today...

Excellent. Greater consistency with a default constructed filebuf is
wonderful, and IMHO would fix DR 435 and libstdc++/10975. Whee!

For:
	* testsuite/27_io/basic_stringbuf/cons/char/1.cc: New.
	* testsuite/27_io/basic_stringbuf/cons/wchar_t/1.cc: Likewise.

Have you thought of adopting the approach that filebuf uses and just use a
constrait_stringbuf. I'd suggest testsuite_io.h....

If you'd like, you could just templatize constraint_filebuf in
testsuite_io.h, rename to constrait_streambuf, inherit from the template
parameter.

-benjamin