Add initial differential fuzzing

Part of #611

fuzz/fuzz_targets/differential.rs

@@ -0,0 +1,13 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use wasmtime_fuzzing::{generators, oracles};
+
+fuzz_target!(|data: (
+    generators::DifferentialConfig,
+    generators::DifferentialConfig,
+    generators::WasmOptTtf
+)| {
+    let (lhs, rhs, wasm) = data;
+    oracles::differential_execution(&wasm, &[lhs, rhs]);
+});