C extension read/write lists of integers is slow

[cheqianh]

Investigate why read/write lists of integers is slow. More specifically, read/write big integers and decimal is slow.

This is kind of duplicated to https://github.com/amzn/ion-python/issues/186 point three, but it's worth to have a separate issue to keep this performance issue on track.

E.g.

results = np.zeros((1000, 80))

# Ion
ion_list_dump = ion.dumps(results.astype("int32").tolist(), binary=True)
ion_list_load = np.array(ion.loads(ion_list_dump))

# JSON 
json_list_dump = json.dumps(results.astype("int32").tolist())
json_list_load = json.loads(json_list_dump)