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/correct/rust-items-preserved.rs

@@ -0,0 +1,30 @@
+#[subxt::subxt(runtime_metadata_path = "../../../artifacts/polkadot_metadata.scale")]
+pub mod node_runtime {
+    pub struct SomeStruct;
+    pub enum SomeEnum {
+        A,
+        B,
+    }
+    pub trait SomeTrait {
+        fn some_func(&self) -> u32;
+    }
+    impl SomeTrait for SomeStruct {
+        fn some_func(&self) -> u32 {
+            1
+        }
+    }
+    impl SomeTrait for SomeEnum {
+        fn some_func(&self) -> u32 {
+            2
+        }
+    }
+}
+
+fn main() {
+    use node_runtime::SomeTrait;
+
+    let unit = node_runtime::SomeStruct;
+    assert_eq!(unit.some_func(), 1);
+    let enumeration = node_runtime::SomeEnum::A;
+    assert_eq!(enumeration.some_func(), 2);
+}