[Rust] Flexbuffers dependency cleanup and fixes (#5998)

* Fix doc comment warnings

Can't use doc comment "///" syntax on macros, that generates the following warning:

warning: unused doc comment
   --> src\flexbuffer_type.rs:236:5
    |
236 |     /// returns true if and only if the flexbuffer type is `VectorFloat4`.
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ rustdoc does not generate documentation for macros
    |
    = help: to document an item produced by a macro, the macro must produce the documentation as part of its expansion

So switched to just use ordinary "//" comments on these to be warning free

* Upgrade num_enum 0.4.1 -> 0.5.0

* Remove unused and non-working usage of test crates

* Remove usage of abandoned debug_stub_derive crate

Which brought in old pre-v1 syn and quote crates.

This replaces it with just manual Debug trait implementation instead for the 2 cases

rust/flexbuffers/src/lib.rs

@@ -12,7 +12,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#![cfg_attr(test, feature(test))]
 //! Flexbuffers is a high performance schemaless binary data format designed at Google.
 //! It is complementary to the schema-ed format [Flatbuffers](http://docs.rs/flatbuffers/).
 //! See [Flexbuffer Internals](https://google.github.io/flatbuffers/flatbuffers_internals.html)
@@ -35,17 +34,8 @@ extern crate byteorder;
 #[macro_use]
 extern crate serde_derive;
 #[macro_use]
-extern crate debug_stub_derive;
 extern crate num_enum;
-#[cfg(test)]
-extern crate quickcheck;
-#[cfg(test)]
-extern crate quickcheck_derive;
-#[cfg(test)]
-extern crate rand;
 extern crate serde;
-#[cfg(test)]
-extern crate test;
 
 mod bitwidth;
 mod builder;