Here is a patch adding the s* format, and changing files, sockets, and
fileio to use it. For bz2, the immediate effect is that you get a type
error (as an object providing bf_releasebuffer cannot be converted
through s#/w# anymore); it would be possible to fix bz2 also to use
s*/w* instead.

I'd like reviewers to focus on flaws in the approach and bugs in the
implementation; existing code should be converted to the new API
afterwards (or not converted at all for 2.6/3.0, but only as patches get
contributed).

If this is accepted in principle, I'll forward-port it to 3.0.