Add fuzzing based on serde_json

This test ensures that we can parse anything that serde_json can
produce, which *ought* to ensure reasonable coverage?

fuzz/Cargo.toml

@@ -8,7 +8,9 @@ edition = "2021"
 cargo-fuzz = true
 
 [dependencies]
+arbitrary = { version = "1.3.2", features = ["derive"] }
 libfuzzer-sys = "0.4"
+serde_json = "1.0.124"
 
 [dependencies.fwd]
 path = ".."
@@ -19,3 +21,10 @@ path = "fuzz_targets/json_raw_input.rs"
 test = false
 doc = false
 bench = false
+
+[[bin]]
+name = "json_only_valid_serde"
+path = "fuzz_targets/json_only_valid_serde.rs"
+test = false
+doc = false
+bench = false