numpy_helper.h alternatives
The build dependencies could be enormously simplified if we could remove the reliance of _libs/src/util.pxd on _libs/src/numpy_helper.h.  I've ported most of the functions directly to util.pxd, would like input on a few remaining ones.

```
PANDAS_INLINE PyObject* get_value_1d(PyArrayObject* ap, Py_ssize_t i) {
    char* item = (char*)PyArray_DATA(ap) + i * PyArray_STRIDE(ap, 0);
    return PyArray_Scalar(item, PyArray_DESCR(ap), (PyObject*)ap);
}
```

It looks like this just returns `ap[i]`.  Presumably more efficient in some way.  I'm guessing this was written before typed memoryviews were available in cython; there may be a less verbose plain-cython alternative available.

Alternatively, everything there except for `PyArray_DESCR` can cimported with a little effort.  If anyone knows of a way to get at this last piece, then both `get_value_1d` and `assign_value_1d` can be moved directly into `util`.
