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.