Forward attributes on type declaration to definition

This'll allow things like `#[derive(Clone)]` or `#[derive(Debug)]` to control
traits for these types.

crates/backend/src/ast.rs

@@ -84,6 +84,7 @@ pub struct ImportStatic {
 pub struct ImportType {
     pub vis: syn::Visibility,
     pub name: Ident,
+    pub attrs: Vec<syn::Attribute>,
 }
 
 #[cfg_attr(feature = "extra-traits", derive(Debug, PartialEq, Eq))]
@@ -469,6 +470,7 @@ impl Program {
         ImportKind::Type(ImportType {
             vis: f.vis,
             name: f.ident,
+            attrs: f.attrs,
         })
     }
 

crates/backend/src/codegen.rs

@@ -505,8 +505,10 @@ impl ToTokens for ast::ImportType {
     fn to_tokens(&self, tokens: &mut TokenStream) {
         let vis = &self.vis;
         let name = &self.name;
+        let attrs = &self.attrs;
         (quote! {
             #[allow(bad_style)]
+            #(#attrs)*
             #vis struct #name {
                 obj: ::wasm_bindgen::JsValue,
             }