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.

.github/workflows/main.yml

@@ -79,7 +79,7 @@ jobs:
     - run: cargo install cargo-fuzz --vers "^0.7"
     - run: cargo fetch
       working-directory: ./fuzz
-    - run: cargo fuzz build --release --debug-assertions
+    - run: cargo fuzz build --release --debug-assertions --features binaryen
     # Our corpora are too large to run in full on every pull request, they just
     # take too long. Instead, we sample some of them and make sure that running
     # our fuzzers over the sampled inputs still works OK.