Fix assert_assignable for array types

Fixes #1226

src/value_and_place.rs

@@ -828,6 +828,9 @@ pub(crate) fn assert_assignable<'tcx>(
                 }
             }
         }
+        (ty::Array(a, _), ty::Array(b, _)) => {
+            assert_assignable(fx, *a, *b)
+        }
         _ => {
             assert_eq!(
                 from_ty, to_ty,