Impl a lifetime-relaxed broadcast for ArrayView

ArrayView::broadcast has a lifetime that depends on &self instead of its
internal buffer. This prevents writing some types of functions in an
allocation-free way. For instance, take the numpy `meshgrid` function:
It could be implemented like so:

```rust
fn meshgrid_2d<'a, 'b>(coords_x: ArrayView1<'a, X>, coords_y: ArrayView1<'b, X>) -> (ArrayView2<'a, X>, ArrayView2<'b, X>) {
    let x_len = coords_x.shape()[0];
    let y_len = coords_y.shape()[0];

    let coords_x_s = coords_x.into_shape((1, y_len)).unwrap();
    let coords_x_b = coords_x_s.broadcast((x_len, y_len)).unwrap();
    let coords_y_s = coords_y.into_shape((x_len, 1)).unwrap();
    let coords_y_b = coords_y_s.broadcast((x_len, y_len)).unwrap();

    (coords_x_b, coords_y_b)
}
```

Unfortunately, this doesn't work, because `coords_x_b` is bound to the
lifetime of `coord_x_s`, instead of being bound to 'a.

This commit introduces a new function, broadcast_ref, that does just
that.

src/impl_views/methods.rs

@@ -0,0 +1,81 @@
+// Copyright 2014-2016 bluss and ndarray developers.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+use crate::imp_prelude::*;
+use crate::dimension::IntoDimension;
+use crate::dimension::size_of_shape_checked;
+
+impl<'a, A, D> ArrayView<'a, A, D>
+where
+    D: Dimension,
+{
+    /// Broadcasts an arrayview.
+    pub fn broadcast_ref<E>(&self, dim: E) -> Option<ArrayView<'a, A, E::Dim>>
+    where
+        E: IntoDimension,
+    {
+        /// Return new stride when trying to grow `from` into shape `to`
+        ///
+        /// Broadcasting works by returning a "fake stride" where elements
+        /// to repeat are in axes with 0 stride, so that several indexes point
+        /// to the same element.
+        ///
+        /// **Note:** Cannot be used for mutable iterators, since repeating
+        /// elements would create aliasing pointers.
+        fn upcast<D: Dimension, E: Dimension>(to: &D, from: &E, stride: &E) -> Option<D> {
+            // Make sure the product of non-zero axis lengths does not exceed
+            // `isize::MAX`. This is the only safety check we need to perform
+            // because all the other constraints of `ArrayBase` are guaranteed
+            // to be met since we're starting from a valid `ArrayBase`.
+            let _ = size_of_shape_checked(to).ok()?;
+
+            let mut new_stride = to.clone();
+            // begin at the back (the least significant dimension)
+            // size of the axis has to either agree or `from` has to be 1
+            if to.ndim() < from.ndim() {
+                return None;
+            }
+
+            {
+                let mut new_stride_iter = new_stride.slice_mut().iter_mut().rev();
+                for ((er, es), dr) in from
+                    .slice()
+                    .iter()
+                    .rev()
+                    .zip(stride.slice().iter().rev())
+                    .zip(new_stride_iter.by_ref())
+                {
+                    /* update strides */
+                    if *dr == *er {
+                        /* keep stride */
+                        *dr = *es;
+                    } else if *er == 1 {
+                        /* dead dimension, zero stride */
+                        *dr = 0
+                    } else {
+                        return None;
+                    }
+                }
+
+                /* set remaining strides to zero */
+                for dr in new_stride_iter {
+                    *dr = 0;
+                }
+            }
+            Some(new_stride)
+        }
+        let dim = dim.into_dimension();
+
+        // Note: zero strides are safe precisely because we return an read-only view
+        let broadcast_strides = match upcast(&dim, &self.dim, &self.strides) {
+            Some(st) => st,
+            None => return None,
+        };
+        unsafe { Some(ArrayView::new(self.ptr, dim, broadcast_strides)) }
+    }
+}

src/impl_views/mod.rs

@@ -1,6 +1,7 @@
 mod constructors;
 mod conversions;
 mod indexing;
+mod methods;
 mod splitting;
 
 pub use constructors::*;