Fix Miri failure with -Zmiri-tag-raw-pointers

Before this commit, running `MIRIFLAGS="-Zmiri-tag-raw-pointers" cargo
miri test test_map_axis` caused Miri to report undefined behavior in
the `test_map_axis` test. This commit fixes the underlying issue.
Basically, Miri doesn't like us using a reference to an element to
access other elements. Here's some sample code to illustrate the
issue:

```rust
let a: Vec<i32> = vec![5, 6];
let first_elt: &i32 = &a[0];
let ptr: *const i32 = first_elt as *const i32;
// Okay: the data is contained in the data referenced by `first_elt`.
let a0 = unsafe { &*ptr.offset(0) };
assert_eq!(*a0, 5);
// Not okay: the data is not contained in the data referenced by `first_elt`.
let a1 = unsafe { &*ptr.offset(1) };
assert_eq!(*a1, 6);
```

Before this commit, we were using `self.index_axis(axis,
0).map(|first_elt| ...)` to create views of the lanes, from references
to the first elements in the lanes. Accessing elements within those
views (other than the first element) led to the Miri error, since the
view's pointer was derived from a reference to a single element.

Thanks to @5225225 for reporting the issue. (This commit fixes rust-ndarray#1137.)

src/impl_methods.rs

@@ -2699,17 +2699,11 @@ where
         A: 'a,
         S: Data,
     {
-        let view_len = self.len_of(axis);
-        let view_stride = self.strides.axis(axis);
-        if view_len == 0 {
+        if self.len_of(axis) == 0 {
             let new_dim = self.dim.remove_axis(axis);
             Array::from_shape_simple_fn(new_dim, move || mapping(ArrayView::from(&[])))
         } else {
-            // use the 0th subview as a map to each 1d array view extended from
-            // the 0th element.
-            self.index_axis(axis, 0).map(|first_elt| unsafe {
-                mapping(ArrayView::new_(first_elt, Ix1(view_len), Ix1(view_stride)))
-            })
+            Zip::from(self.lanes(axis)).map_collect(mapping)
         }
     }
 
@@ -2730,21 +2724,11 @@ where
         A: 'a,
         S: DataMut,
     {
-        let view_len = self.len_of(axis);
-        let view_stride = self.strides.axis(axis);
-        if view_len == 0 {
+        if self.len_of(axis) == 0 {
             let new_dim = self.dim.remove_axis(axis);
             Array::from_shape_simple_fn(new_dim, move || mapping(ArrayViewMut::from(&mut [])))
         } else {
-            // use the 0th subview as a map to each 1d array view extended from
-            // the 0th element.
-            self.index_axis_mut(axis, 0).map_mut(|first_elt| unsafe {
-                mapping(ArrayViewMut::new_(
-                    first_elt,
-                    Ix1(view_len),
-                    Ix1(view_stride),
-                ))
-            })
+            Zip::from(self.lanes_mut(axis)).map_collect(mapping)
         }
     }