PERF: lookups on Timestamp attributes
This is pretty ugly, but it avoids python-space attribute lookups e.g. `__Pyx_PyObject_GetAttrStr(((PyObject *)__pyx_v_self), __pyx_n_s_microsecond)`

cc @scoder is there a way to make this unnecessary?  I'm imagining a cdef property that aliased these attributes appropriately.