Fix clippy warnings across workspace

- io.rs: factor the pending-RPC-replies map into a PendingReplies
  type alias (type_complexity)
- lua_env.rs: drop a redundant i64->i64 cast and an unneeded borrow
  before create_string (unnecessary_cast, needless_borrows_for_generic_args)
- state_engine.rs: iterate watches.values() instead of discarding the
  key from watches.iter() (for_kv_map)
- module_host_sandbox.rs: collapse nested if into a single condition
  (collapsible_if)

(cherry picked from commit 49063cb98ddfeae35d842a6cf439123cfda763d9)

breadd/tests/module_host_sandbox.rs

@@ -137,10 +137,8 @@ enabled = false
     async fn wait_until_ready(&self) -> Result<()> {
         let deadline = Instant::now() + Duration::from_secs(8);
         while Instant::now() < deadline {
-            if self.socket_path.exists() {
-                if self.send_request("ping", json!({})).await.is_ok() {
-                    return Ok(());
-                }
+            if self.socket_path.exists() && self.send_request("ping", json!({})).await.is_ok() {
+                return Ok(());
             }
             sleep(Duration::from_millis(100)).await;
         }