Fix clippy::toplevel_ref_arg at stack! callsites

`let x = &y` is preferred over `let ref x = y`

Author: CattleProdigy

nalgebra-macros/src/stack_impl.rs

@@ -26,7 +26,7 @@ pub fn stack_impl(matrix: Matrix) -> syn::Result<TokenStream2> {
                 let ident_block = format_ident!("{prefix}_stack_{i}_{j}_block");
                 let ident_shape = format_ident!("{prefix}_stack_{i}_{j}_shape");
                 output.extend(std::iter::once(quote_spanned! {expr.span()=>
-                    let ref #ident_block = #expr;
+                    let #ident_block = &(#expr);
                     let #ident_shape = #ident_block.shape_generic();
                 }));
             }