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/Cargo.toml

@@ -34,15 +34,21 @@ name = "instantiate_translated"
 path = "fuzz_targets/instantiate_translated.rs"
 test = false
 doc = false
+required-features = ['binaryen']
 
 [[bin]]
 name = "api_calls"
 path = "fuzz_targets/api_calls.rs"
 test = false
 doc = false
+required-features = ['binaryen']
 
 [[bin]]
 name = "differential"
 path = "fuzz_targets/differential.rs"
 test = false
 doc = false
+required-features = ['binaryen']
+
+[features]
+binaryen = ['wasmtime-fuzzing/binaryen']