[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/reader/map.rs

@@ -22,9 +22,8 @@ use std::iter::{DoubleEndedIterator, ExactSizeIterator, FusedIterator, Iterator}
 /// MapReaders may be indexed with strings or usizes. `index` returns a result type,
 /// which may indicate failure due to a missing key or bad data, `idx` returns an Null Reader in
 /// cases of error.
-#[derive(DebugStub, Default, Clone)]
+#[derive(Default, Clone)]
 pub struct MapReader<'de> {
-    #[debug_stub = "&[..]"]
     pub(super) buffer: &'de [u8],
     pub(super) values_address: usize,
     pub(super) keys_address: usize,
@@ -33,6 +32,20 @@ pub struct MapReader<'de> {
     pub(super) length: usize,
 }
 
+// manual implementation of Debug because buffer slice can't be automatically displayed
+impl<'de> std::fmt::Debug for MapReader<'de> {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        // skips buffer field
+        f.debug_struct("MapReader")
+            .field("values_address", &self.values_address)
+            .field("keys_address", &self.keys_address)
+            .field("values_width", &self.values_width)
+            .field("keys_width", &self.keys_width)
+            .field("length", &self.length)
+            .finish()
+    }
+}
+
 impl<'de> MapReader<'de> {
     /// Returns the number of key/value pairs are in the map.
     pub fn len(&self) -> usize {