[naga] Use Handle::new, don't construct the Handle directly.

In preparation for making `Handle` fields private to another module,
have the `Iterator` implementation for `arena::Range` call
`Handle::new` to construct the handle being produced, rather than
building it using a struct literal.

naga/src/arena.rs

@@ -185,10 +185,7 @@ impl<T> Iterator for Range<T> {
         if self.inner.start < self.inner.end {
             let next = self.inner.start;
             self.inner.start += 1;
-            Some(Handle {
-                index: NonMaxU32::new(next).unwrap(),
-                marker: self.marker,
-            })
+            Some(Handle::new(NonMaxU32::new(next).unwrap()))
         } else {
             None
         }