Making unwind exceptions work with primitives.

When primitives functions are in any position in the frame stack, the
old unwind procedure would not check for it, thus:

-not giving a chance for primitives to catch exceptions
-probably screwing up the synchrony of c stack and memetalk stack (eg. a
 primitive calling do_send/call() in the middle of the stack could
 receive the actual /program/'s result -- main's return value -- if an
 exception is thrown crossing over primitive do_sends and being caught
 in the bottom level).

Closes #131