rustc_metadata: Switch all decoder methods from vectors to iterators

Also remove unnecessary `is_proc_macro_crate` checks from decoder

compiler/rustc_resolve/src/diagnostics.rs

@@ -914,11 +914,8 @@ impl<'a> Resolver<'a> {
                             // a note about editions
                             let note = if let Some(did) = did {
                                 let requires_note = !did.is_local()
-                                    && this
-                                        .cstore()
-                                        .item_attrs_untracked(did, this.session)
-                                        .iter()
-                                        .any(|attr| {
+                                    && this.cstore().item_attrs_untracked(did, this.session).any(
+                                        |attr| {
                                             if attr.has_name(sym::rustc_diagnostic_item) {
                                                 [sym::TryInto, sym::TryFrom, sym::FromIterator]
                                                     .map(|x| Some(x))
@@ -926,7 +923,8 @@ impl<'a> Resolver<'a> {
                                             } else {
                                                 false
                                             }
-                                        });
+                                        },
+                                    );
 
                                 requires_note.then(|| {
                                     format!(