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/ocaml/Makefile

@@ -0,0 +1,33 @@
+# Build a library allowing FFI access to the Wasm spec interpreter.
+
+OCAML_FLAGS := -g -keep-locs -runtime-variant _pic
+# By default, we build in a sub-directory but we can override this with `make
+# BUILD_DIR=...`.
+BUILD_DIR := _build
+# Currently the WebAssembly spec interpreter is buried in a Git submodule as is
+# its build directory, `_build`. Cargo may not like that files are changing
+# outside of `target` (TODO).
+SPEC_DIR := spec/interpreter
+SPEC_BUILD_DIR := $(SPEC_DIR)/_build
+SPEC_LIB := $(SPEC_BUILD_DIR)/wasm.cmxa
+
+
+# Build and package the static library, `libinterpret.a`.
+$(BUILD_DIR)/libinterpret.a: $(BUILD_DIR)/interpret.lib.o
+	ar qs $@ $^
+$(BUILD_DIR)/interpret.lib.o: $(SPEC_LIB) $(BUILD_DIR)/interpret.cmx
+	ocamlopt $(OCAML_FLAGS) -I $(SPEC_BUILD_DIR) -o $@ -output-complete-obj $^
+$(BUILD_DIR)/interpret.cmx: interpret.ml $(SPEC_BUILD_DIR) $(BUILD_DIR)
+	ocamlopt $(OCAML_FLAGS) -I $(SPEC_BUILD_DIR) -o $@ -c -impl $<
+$(BUILD_DIR):
+	mkdir -p $@
+
+
+# We also need to be able to build the spec's `wasm.cmxa`.
+$(SPEC_LIB):
+	make -C $(SPEC_DIR) libopt
+
+
+clean:
+	rm -rf $(BUILD_DIR)
+	make -C $(SPEC_DIR) clean