Feedback on __mt_alloc?

[B. Kosnik]

>I don't think we really gain anything by splitting them up, if we have
>to go to each bits/* header and re-include them.  Or by including all the
>new ones back into stl_alloc.h.

My thought was that they would be moved to ext, and only included and
used when so desired. Hopefully the default allocator mechanism could be
simplified as well. Separating the allocators out would also clarify the
licensing on newer allocators.

In addition, I'm hoping that GLIBCPP_FORCE_NEW could be generalized to
switch between more than the __pool_alloc and __malloc_alloc allocators.
Something like GLIBCPP_ALLOC = x. I could imagine this being useful.

-benjamin