Minimize <iosfwd>

[Paolo Carlini]

Benjamin Kosnik wrote:

>> I'd be happy to make changes to the mbstate_t handling, but I'm going
>> to need more guidance.  I am not sufficiently familiar with the
>> library organization or configuration logic to understand what you
>> want done from the above description.
>
> Sure. Hang on...

Excellent. Benjamin, I just went through the headers including <iosfwd> 
and I'm pretty sure that removing from the latter <bits/c++locale.h> is 
ok, basically because <bits/localefwd.h> takes care of including it 
anyway. Very worst case, one/two headers may require adjustements. 
That's very nice, because <bits/c++locale.h> alone is about two thirds 
of <iosfwd>.

Remains to be checked <bits/functexcept.h>, which is small anyway, and 
finally the most delicate part, <bits/postypes.h>, but we could also 
proceed incrementally, or better said, DEcrementally ;)

Paolo.