Split our existing fuzz targets into separate generators and oracles

Part of #611

fuzz/fuzz_targets/instantiate.rs

@@ -2,28 +2,10 @@
 
 extern crate libfuzzer_sys;
 
-use cranelift_codegen::settings;
 use libfuzzer_sys::fuzz_target;
-use wasmparser::validate;
-use wasmtime_jit::{instantiate, CompilationStrategy, Compiler, NullResolver};
+use wasmtime_fuzzing::oracles;
+use wasmtime_jit::{CompilationStrategy};
 
 fuzz_target!(|data: &[u8]| {
-    if validate(data, None).is_err() {
-        return;
-    }
-    let flag_builder = settings::builder();
-    let isa_builder = cranelift_native::builder().unwrap_or_else(|_| {
-        panic!("host machine is not a supported target");
-    });
-    let isa = isa_builder.finish(settings::Flags::new(flag_builder));
-    let mut compiler = Compiler::new(isa, CompilationStrategy::Auto);
-    let mut imports_resolver = NullResolver {};
-    let _instance = instantiate(
-        &mut compiler,
-        data,
-        &mut imports_resolver,
-        Default::default(),
-        true,
-    )
-    .unwrap();
+    oracles::instantiate(data, CompilationStrategy::Auto);
 });