Tweak how we create raw views in accumulate_axis_inplace

We had:

1. let ptr1 = self.raw_view(); // Borrow &self
2. let ptr2 = self.raw_view_mut(); // Borrow &mut self
3. Use ptr1 and ptr2

While I'm not an expert at the unsafe coding guidelines for Rust, and
there are more places in ndarray to revisit, I think it's best to change
change 1 and 2 - they don't pass my internalized borrow checker.

It seems as though the steps 1, 2, 3 could be against the rules as ptr1
is borrowed from the array data, and its scope straddles the mut borrow
of the array data in 2.

For this reason, I think this would be better:

1. let ptr2 = self.raw_view_mut() // Borrow &mut self
2. let ptr1 = derive from ptr2
3. use ptr1 and ptr2

RawView should hopefully be our ally in making a better ndarray from the
foundation.

Author: bluss

src/impl_methods.rs

@@ -2268,9 +2268,9 @@ where
         if self.len_of(axis) <= 1 {
             return;
         }
-        let mut prev = self.raw_view();
+        let mut curr = self.raw_view_mut(); // mut borrow of the array here
+        let mut prev = curr.raw_view();     // derive further raw views from the same borrow
         prev.slice_axis_inplace(axis, Slice::from(..-1));
-        let mut curr = self.raw_view_mut();
         curr.slice_axis_inplace(axis, Slice::from(1..));
         // This implementation relies on `Zip` iterating along `axis` in order.
         Zip::from(prev).and(curr).apply(|prev, curr| unsafe {