Retain Rust code items from mod decorated with subxt attribute (#721)

* refactor: Simplify collecting type substitutes

* Simplify ItemMod::from

* Preserve inner Rust items when expanding the subxt::subxt macro

* No named lifetimes

* Update codegen/src/api/mod.rs

Co-authored-by: Andrew Jones <ascjones@gmail.com>

* Move passing UI tests under `ui_tests`

Co-authored-by: Andrew Jones <ascjones@gmail.com>

codegen/src/ir.rs

@@ -26,16 +26,13 @@ impl From<syn::ItemMod> for ItemMod {
                 abort!(module, "out-of-line subxt modules are not supported",)
             }
         };
-        let items = items
-            .into_iter()
-            .map(<Item as From<syn::Item>>::from)
-            .collect::<Vec<_>>();
+
         Self {
             vis: module.vis,
             mod_token: module.mod_token,
             ident: module.ident,
             brace,
-            items,
+            items: items.into_iter().map(From::from).collect(),
         }
     }
 }
@@ -57,6 +54,10 @@ impl ItemMod {
             })
             .collect()
     }
+
+    pub fn rust_items(&self) -> impl Iterator<Item = &syn::Item> {
+        self.items.iter().filter_map(Item::as_rust)
+    }
 }
 
 #[allow(clippy::large_enum_variant)]
@@ -66,6 +67,15 @@ pub enum Item {
     Subxt(SubxtItem),
 }
 
+impl Item {
+    pub fn as_rust(&self) -> Option<&syn::Item> {
+        match self {
+            Item::Rust(item) => Some(item),
+            _ => None,
+        }
+    }
+}
+
 impl From<syn::Item> for Item {
     fn from(item: syn::Item) -> Self {
         if let syn::Item::Use(ref use_) = item {