Use int128 (really LONG integers) in cython

_Very_big_int_in_cython.txt

Taken from https://stackoverflow.com/questions/27582001/how-to-use-128-bit-integers-in-cython

Basically, we make cython believe we will use 64 bit int to generate the .c file,
but by using a C header we will, in fact, define a 128 bit int (the definition
in the .h file does not match what we put into the pyx file).

Once Cython has generated the .c file, we can compile it with GCC without further trouble,
as GCC does support 128 bit ints

foo.pyx

cdef extern from "header_int128.h":
    # Basically, we cheat cython to think we will use unsigned long long,
    # which is a 64 bit int ...
    ctypedef unsigned long long int128

print "hello world"

cpdef int foo():
    cdef int128 foo = 4
    return 32

header_int128.h

// ... but in the header, we give another definition,
// the one for a 128 bit int.
typedef __int128_t int128;