Rust Flexbuffers (#5669)

* Cargo clippy lints

* more lints

* more lints

* Restored a doc comment

* Comment on float eps-eq and adjusted casting

* Rust Flexbuffers

* more serde tests, removed some unsafe

* Redid serde to be map-like and Reader is Display

* Moved iter from Reader to VectorReader

* Serious quickcheck + bugs

* wvo api

* Made types smaller for a reasonable speedup

* redid reading in a way that's a bit faster.

Profiling shows the rust slowdown as building +10%, reading +20%

* src/bin are developer binaries in rust

* Root and Map width are not packed

* key null check is debug only + doc changes

* BuilderOptions

* Documentation

* Documentation

* Moved tests to rust_usage_test

* Moved rust flexbuffers samples to Flatbuffers/samples

* Fixed RustTest

* Fixed for Rust 1.37.0

* Upgraded to rust 1_40_0

* fixed a little-endian-only feature in a test

* 1.40.0

* fixed some benchmarks for bigendian

* Updated .bat file

* misspelling

* Gold Flexbuffer test.

* Serialize,Deserialize, std::error::Error for Errors.

* Undo rustfmt in integration_test.rs

* from_slice instead of from_vec

* Added comments to unsafe blocks

* expanded on comment

* bump

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

tests/rust_usage_test/tests/integration_test.rs

@@ -15,9 +15,18 @@
  * limitations under the License.
  */
 
+#[macro_use]
 extern crate quickcheck;
-
 extern crate flatbuffers;
+extern crate flexbuffers;
+extern crate rand;
+extern crate serde;
+#[macro_use]
+extern crate serde_derive;
+#[macro_use]
+extern crate quickcheck_derive;
+
+mod flexbuffers_tests;
 
 #[allow(dead_code, unused_imports)]
 #[path = "../../include_test/include_test1_generated.rs"]
@@ -32,6 +41,11 @@ pub mod include_test2_generated;
 mod monster_test_generated;
 pub use monster_test_generated::my_game;
 
+#[rustfmt::skip] // TODO: Use standard rust formatting and remove dead code.
+#[allow(dead_code)]
+mod flatbuffers_tests {
+use super::*;
+
 // Include simple random number generator to ensure results will be the
 // same across platforms.
 // http://en.wikipedia.org/wiki/Park%E2%80%93Miller_random_number_generator
@@ -2907,3 +2921,4 @@ fn load_file(filename: &str) -> Result<Vec<u8>, std::io::Error> {
     f.read_to_end(&mut buf)?;
     Ok(buf)
 }
+}