cargo fuzz integration (#306)

* added wip translate_module fuzzer

* use local binaryen-rs fork (with shim) for fuzzing

* minor doc cleanup

* check fuzzer integration via CI

* switch back to upstream binaryen-rs; add forgotten integration test directive

cranelift/FUZZING.md

@@ -0,0 +1,9 @@
+# Fuzzing
+
+This document describes how to fuzz cretonne with [`cargo-fuzz`](https://github.com/rust-fuzz/cargo-fuzz). The fuzz targets use `wasm-opt` from [`binaryen-rs`](https://github.com/pepyakin/binaryen-rs) to generate valid WebAssembly modules from the fuzzed input supplied by `cargo-fuzz` (via [libfuzzer](http://llvm.org/docs/LibFuzzer.html)). In this scheme coverage feedback from both cretonne and the `wasm-opt` input generation code is used to inform the fuzzer.
+
+# Usage
+
+1. Install all dependencies required to build `binaryen-rs` and `cargo-fuzz` (including `cmake`)
+2. Use the rust nightly toolchain (required by `cargo-fuzz`): `rustup override set nightly`
+3. Execute the fuzz target: `cargo fuzz run fuzz_translate_module`