Updated callbacks

Hi Guys -

Just FYI, I found a major bug in the callback support I posted earlier.
The problem occurs if one fails a primitive *after* a callback has been
invoked like here:

Squeak -> (primitive) -> Plugin
                          Plugin -> (callback) -> Squeak
                          Plugin <-  (return)  <- Squeak
Squeak <-  (failure)  <- Plugin

In such a case, the method being activated by the primitive failure is
not the method originally invoking the primitive but rather the one that
returned from the callback. More technically speaking, the interpreter's
newMethod isn't preserved which causes the effect described above. The
attached version fixes the problem by preserving newMethod across
callback enter/return.

Cheers,
   - Andreas