Rust: remove inner attributes (#6410)

* remove inner attributes

* Added test for outdir in Rust

* add bin/outdir

* Moved outdir test to its own package and only run it if flatc is available

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

samples/monster_generated.rs

@@ -1,7 +1,6 @@
 // automatically generated by the FlatBuffers compiler, do not modify
 
 
-#![allow(unused_imports, dead_code)]
 
 use std::mem;
 use std::cmp::Ordering;

samples/sample_binary.rs

@@ -18,6 +18,7 @@
 extern crate flatbuffers;
 
 // import the generated code
+#[allow(dead_code, unused_imports)]
 #[path = "./monster_generated.rs"]
 #[allow(clippy::approx_constant)]  // We use low precision PI as a default value.
 mod monster_generated;