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/src/lib.rs

@@ -0,0 +1,33 @@
+//! This library provides a way to interpret Wasm functions in the official Wasm
+//! specification interpreter, written in OCaml, from Rust.
+//!
+//! In order to not break Wasmtime's build, this library will always compile. It
+//! does depend on certain tools (see `README.md`) that may or may not be
+//! available in the environment:
+//!  - when the tools are available, we build and link to an OCaml static
+//!    library (see `with_library` module)
+//!  - when the tools are not available, this library will panic at runtime (see
+//!    `without_library` module).
+
+/// Enumerate the kinds of Wasm values.
+#[derive(Clone, Debug, PartialEq)]
+pub enum Value {
+    I32(i32),
+    I64(i64),
+    F32(i32),
+    F64(i64),
+}
+
+#[cfg(feature = "has-libinterpret")]
+mod with_library;
+#[cfg(feature = "has-libinterpret")]
+pub use with_library::*;
+
+#[cfg(not(feature = "has-libinterpret"))]
+mod without_library;
+#[cfg(not(feature = "has-libinterpret"))]
+pub use without_library::*;
+
+// If the user is fuzzing`, we expect the OCaml library to have been built.
+#[cfg(all(fuzzing, not(feature = "has-libinterpret")))]
+compile_error!("The OCaml library was not built.");