Is there a nicer way to pass python long ints (bigint) into C
efficiently?  I'm currently cutting the value up into 64 bit chunks in
python and passing in as an unsigned long*:

cdef("int bigInt(int n, unsigned long* x);")

x = sum(1 << i for i in [100,200,123])
xs = []
while x > 0:
    xs.append(x & ((1 << 64) - 1))
    x >>= 64
print lib.bigInt(len(xs), xs)

but this is quite slow, and it seems like this data must already be
lurking somewhere in exactly the right form.