Implement Serialize on generated rust types (#7022)

* fix for rust build

* Rust: Implement Serialize on generated types

For debugging convenience it is really handy to be able to dump out
types as another format (ie: json). For example, if we are logging a
type to a structured logging system, or even printing it out in a
structured way to the console.

Right now we handle this by shelling out to `flatc` which is not ideal;
by implementing Serialize on the generated types we can use any of the
Serializer-implementing packages for our structured debug output.

* clang-format

* Make the flatbuffers Rust crate only have an optional dependency on the `serde` packages.

* fix warning

* fix rust test build

* Oh yeah this needs to be initialized

* fix toml syntax

* code review feedback

* rebuild test data

rust/flatbuffers/src/array.rs

@@ -133,3 +133,22 @@ where
         array.assume_init()
     }
 }
+
+#[cfg(feature="serialize")]
+impl<'a, T: 'a, const N: usize> serde::ser::Serialize for Array<'a, T, N>
+where
+    T: 'a + Follow<'a>,
+    <T as Follow<'a>>::Inner: serde::ser::Serialize,
+{
+    fn serialize<S>(&self, serializer: S) -> std::result::Result<S::Ok, S::Error>
+    where
+        S: serde::ser::Serializer,
+    {
+        use serde::ser::SerializeSeq;
+        let mut seq = serializer.serialize_seq(Some(self.len()))?;
+        for element in self.iter() {
+            seq.serialize_element(&element)?;
+        }
+        seq.end()
+    }
+}