Mark endian_scalar as unsafe. (#6588)

* Mark endian_scalar as unsafe.

Also
- removed the deprecated flexbuffer slice from example
- fixed some cargo warnings

* Assertions and read_scalar made unsafe

* Clippy lints

* Add to Safety

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

tests/rust_usage_test/tests/flexbuffers_tests/rwyw.rs

@@ -255,6 +255,7 @@ fn vector_uint4() {
     assert_eq!(v.idx(2).get_u64(), Ok(5));
     assert_eq!(v.idx(3).get_u64(), Ok(7));
     assert!(v.index(4).is_err());
+    #[allow(deprecated)]
     #[cfg(target_endian = "little")]
     {
         assert_eq!(r.get_slice::<u8>().unwrap(), [2, 3, 5, 7]);
@@ -399,7 +400,7 @@ fn serde_serious() {
         b: u16,
         c: u32,
         d: u64,
-    };
+    }
 
     #[derive(Debug, PartialEq, Serialize, Deserialize)]
     struct MyUnitStruct(Vec<String>);