Added PyErr_MemoryErrorInst to hold the pre-instantiated instance when
using class based exceptions.

Simplified the creation of all built-in exceptions, both class based
and string based.  Actually, for class based exceptions, the string
ones are still created just in case there's a problem creating the
class based ones (so you still get *some* exception handling!).  Now
the init and fini functions run through a list of structure elements,
creating the strings (and optionally classes) for every entry.

initerrors(): the new base class exceptions StandardError,
LookupError, and NumberError are initialized when using string
exceptions, to tuples containing the list of derived string
exceptions.  This GvR trick enables forward compatibility!  One bit of
nastiness is that the C code has to know the inheritance tree embodied
in exceptions.py.

Added the two phase init and fini functions.