Decref cdef globals, interned values, and imported types

Temporary variables must be set to zero or they might be XDECREF'ed later, generating very hard to track bugs.

Fix __Pyx_GetExcValue to avoid (possibly) running Python code while the thread state is inconsistent

For example, in the SAGE source we have

1158 PyInt_FromLong(0)
 776 PyInt_FromLong(1)
 258 PyInt_FromLong(2)
  33 PyInt_FromLong(3)
  21 PyInt_FromLong(10)

and a thousand or so others... Who knows how many of these are in loops too.

This is what the interpreter does, and allows one to
get at the actual object (rather than just its truth
value).