Turn off binaryen in fuzzing by default

... but turn it back on in CI by default. The `binaryen-sys` crate
builds binaryen from source, which is a drag on CI for a few reasons:

* This is quite large and takes a good deal of time to build
* The debug build directory for binaryen is 4GB large

In an effort to both save time and disk space on the builders this
commit adds a `binaryen` feature to the `wasmtime-fuzz` crate. This
feature is enabled specifically when running the fuzzers on CI, but it
is disabled during the typical `cargo test --all` command. This means
that the test builders should save an extra 4G of space and be a bit
speedier now that they don't build a giant wad of C++.

We'll need to update the OSS-fuzz integration to enable the `binaryen`
feature when executing `cargo fuzz build`, and I'll do that once this
gets closer to landing.

crates/fuzzing/src/oracles.rs

@@ -110,7 +110,7 @@ pub fn compile(wasm: &[u8], strategy: Strategy) {
 /// or aren't enabled for different configs, we should get the same results when
 /// we call the exported functions for all of our different configs.
 pub fn differential_execution(
-    ttf: &crate::generators::WasmOptTtf,
+    wasm: &[u8],
     configs: &[crate::generators::DifferentialConfig],
 ) {
     crate::init_fuzzing();
@@ -131,13 +131,13 @@ pub fn differential_execution(
     };
 
     let mut export_func_results: HashMap<String, Result<Box<[Val]>, Trap>> = Default::default();
-    log_wasm(&ttf.wasm);
+    log_wasm(wasm);
 
     for config in &configs {
         let engine = Engine::new(config);
         let store = Store::new(&engine);
 
-        let module = match Module::new(&store, &ttf.wasm) {
+        let module = match Module::new(&store, wasm) {
             Ok(module) => module,
             // The module might rely on some feature that our config didn't
             // enable or something like that.
@@ -283,6 +283,7 @@ fn assert_same_export_func_result(
 }
 
 /// Invoke the given API calls.
+#[cfg(feature = "binaryen")]
 pub fn make_api_calls(api: crate::generators::api::ApiCalls) {
     use crate::generators::api::ApiCall;