[v3] symbol versioning fixups

[Paolo Carlini]

Paolo Carlini wrote:

>Benjamin Kosnik wrote:
>
>>	* src/istream.cc: Move istream::ignore(streamsize) specializations...
>>	* src/compatibility.cc: ...here. New.
>>
>Also, I don't understand this part: ignore(streamsize) is a *new* symbol
>- previously we had *only* the version with two arguments (and
>defaults): it's needed for the new library not for compatibility with
>the old one. Is this change really necessary?
>  
>
Ok, sorry, disregard this: I got sidetracked by the word
"compatibility", which is actually about 3.4.0 vs 3.4.5, not about 3.4.x
vs 4.0.0.

Actually, now I see more clearly that this whole issue is about the
above delicate point, as explained by Benjamin in the second paragraph,
not about forward compatibility per se between 3.4 and 4.0, which is OK
anyway.

To summarize, the only minor thing which is still not clear to me is
char_traits::eq...

Paolo.