Add a spec test fuzzer for Config (#1509)

* Add a spec test fuzzer for Config

This commit adds a new fuzzer which is intended to run on oss-fuzz. This
fuzzer creates and arbitrary `Config` which *should* pass spec tests and
then asserts that it does so. The goal here is to weed out any
accidental bugs in global configuration which could cause
non-spec-compliant behavior.

* Move implementation to `fuzzing` crate

crates/fuzzing/build.rs

@@ -0,0 +1,26 @@
+// A small build script to include the contents of the spec test suite into the
+// final fuzzing binary so the fuzzing binary can be run elsewhere and doesn't
+// rely on the original source tree.
+
+use std::env;
+use std::path::PathBuf;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
+    let dir = env::current_dir()
+        .unwrap()
+        .join("../../tests/spec_testsuite");
+    let mut code = format!("static FILES: &[(&str, &str)] = &[\n");
+    for entry in dir.read_dir().unwrap() {
+        let entry = entry.unwrap();
+        let path = entry.path().display().to_string();
+        if !path.ends_with(".wast") {
+            continue;
+        }
+        code.push_str(&format!("({:?}, include_str!({0:?})),\n", path));
+    }
+    code.push_str("];\n");
+    std::fs::write(out_dir.join("spectests.rs"), code).unwrap();
+}