Use NumPy to get the pointers of the memoryviews and do pointer arithmetic. This is way more reliable and self-contained.

TODO unit test for `merge_memoryviews` and deal with NumPy situation

It's very annoying: `from_buffer` requires the buffer to be writeable, because ctypes is inconsistently strict about const correctness (see https://bugs.python.org/issue11427#msg148586). I don't think there's any clever pointer-casting we could do as mentioned in that thread, since we need to access through the buffer protocol—we can't just cast `mv.obj` to `c_void_p`, since we need the pointer into the current location within the buffer, not just at the start of the buffer.

So for read-only buffers, we try to use NumPy, and if it's not available, we'll end up silently(!) falling back on copying.

Co-authored-by: crusaderky <[email protected]>

Co-authored-by: jakirkham <[email protected]>

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