Turn on clippy for Rust and fix lints for non-generated code (#7575)

Co-authored-by: Casper Neo <cneo@google.com>

tests/rust_usage_test/bin/monster_example.rs

@@ -1,5 +1,7 @@
+#![allow(clippy::derivable_impls, clippy::all)]
 extern crate flatbuffers;
 
+
 #[allow(dead_code, unused_imports)]
 #[path = "../../include_test1/mod.rs"]
 pub mod include_test1_generated;