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.

fuzz/fuzz_targets/differential.rs

@@ -9,5 +9,5 @@ fuzz_target!(|data: (
     generators::WasmOptTtf
 )| {
     let (lhs, rhs, wasm) = data;
-    oracles::differential_execution(&wasm, &[lhs, rhs]);
+    oracles::differential_execution(&wasm.wasm, &[lhs, rhs]);
 });