Cythonize `Buffer`

From @shwina 
```python
In [15]: mr = DeviceMemoryResource(Device().device_id)

In [16]: %timeit mr.allocate(10)
3.35 μs ± 95.1 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

In [17]: %timeit cp.empty(10, dtype=cp.int8)
1.48 μs ± 11.4 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
```