Add a crate to interface with the WebAssembly spec interpreter

The WebAssembly spec interpreter is written in OCaml and the new crate
uses `ocaml-interop` along with a small OCaml wrapper to interpret Wasm
modules in-process. The build process for this crate is currently
Linux-specific: it requires several OCaml packages (e.g. `apt install -y
ocaml-nox ocamlbuild`) as well as `make`, `cp`, and `ar`.

crates/fuzzing/wasm-spec-interpreter/README.md

@@ -0,0 +1,39 @@
+wasm-spec-interpreter
+=====================
+
+This project shows how to use `ocaml-interop` to call into the Wasm spec
+interpreter. There are several steps to making this work:
+ - building the OCaml Wasm spec interpreter as a static library
+ - building a Rust-to-OCaml FFI bridge using `ocaml-interop` and a custom OCaml
+   wrapper
+ - linking both things into a Rust crate
+
+### Dependencies
+
+This crate only builds in an environment with:
+- `make` (the Wasm spec interpreter uses a `Makefile`)
+- `ocamlopt`, `ocamlbuild` (available with, e.g., `dnf install ocaml`)
+- Linux tools (e.g. `ar`); currently it is easiest to build the static
+  libraries in a single environment but this could be fixed in the future (TODO)
+
+Remember to retrieve the Wasm spec submodule:
+
+```
+git clone ... --recursive
+```
+
+### Build
+
+```
+RUSTFLAGS=--cfg=fuzzing cargo build
+```
+
+Use `FFI_LIB_DIR=path/to/lib/...` to specify a different location for the static
+library (this is mainly for debugging). If the `--cfg=fuzzing` configuration is
+not provided, this crate will build successfully but fail at runtime.
+
+### Test
+
+```
+RUSTFLAGS=--cfg=fuzzing cargo test
+```