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>

testing/ui-tests/src/lib.rs

@@ -25,6 +25,8 @@ fn ui_tests() {
     let mut m = MetadataTestRunner::default();
     let t = trybuild::TestCases::new();
 
+    t.pass("src/correct/*.rs");
+
     // Check that storage maps with no keys are handled properly.
     t.pass(&m.path_to_ui_test_for_metadata(
         "storage_map_no_keys",