move the flatten-serde-json crate inside of milli

flatten-serde-json/fuzz/fuzz_targets/flatten.rs

@@ -0,0 +1,8 @@
+#![no_main]
+use arbitrary_json::ArbitraryObject;
+use flatten_serde_json::flatten;
+use libfuzzer_sys::fuzz_target;
+
+fuzz_target!(|object: ArbitraryObject| {
+    let _ = flatten(&object);
+});