[v3] symbol versioning fixups

[Paolo Carlini]

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?

Paolo.