Bump minimal supported NaCl version.

We need pp::Instance::LogToConsole(), which isn't available in pepper_22. I
don't know whether it is supported in 23 or 24, but in 25 it is and
currently 26 is the oldest supported version, do I didn't bother.
Utility: use `nullptr` instead of `0`.
Utility: added NaCl{Console,Message}StreamBuffer classes.

Usable for redirecting stream output to JavaScript console or sending
them via message event to JavaScript.