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

tests/rust_usage_test/Cargo.toml

@@ -44,6 +44,7 @@ path = "../../samples/sample_binary.rs"
 [dev-dependencies]
 quickcheck = "0.6"
 # TODO(rw): look into moving to criterion.rs
+serde = "1.0"
 bencher = "0.1.5"
 static_assertions = "1.0.0"
 rand = "*"

tests/rust_usage_test/outdir/Cargo.toml

@@ -8,3 +8,4 @@ edition = "2018"
 
 [dependencies]
 flatbuffers = { path = "../../../rust/flatbuffers" }
+serde = "1.0"