Implement Serialize for flexbuffer::Reader (#6635)

* Implement Serialize for flexbuffer::Reader

* bump version

* Remove use of experimantal or-patterns

* Remove more use of experimantal or-patterns

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

rust/flexbuffers/src/reader/vector.rs

@@ -40,7 +40,7 @@ impl<B: Buffer> Default for VectorReader<B> {
     fn default() -> Self {
         VectorReader {
             reader: Reader::default(),
-            length: usize::default()
+            length: usize::default(),
         }
     }
 }