fuzzing: Add initial API call fuzzer

We only generate *valid* sequences of API calls. To do this, we keep track of
what objects we've already created in earlier API calls via the `Scope` struct.

To generate even-more-pathological sequences of API calls, we use [swarm
testing]:

> In swarm testing, the usual practice of potentially including all features
> in every test case is abandoned. Rather, a large “swarm” of randomly
> generated configurations, each of which omits some features, is used, with
> configurations receiving equal resources.

[swarm testing]: https://www.cs.utah.edu/~regehr/papers/swarm12.pdf

There are more public APIs and instance introspection APIs that we have than
this fuzzer exercises right now. We will need a better generator of valid Wasm
than `wasm-opt -ttf` to really get the most out of those currently-unexercised
APIs, since the Wasm modules generated by `wasm-opt -ttf` don't import and
export a huge variety of things.

crates/fuzzing/src/lib.rs

@@ -107,8 +107,12 @@ fn my_fuzzing_regression_test() {{
     }
 }
 
-fn scratch_dir() -> PathBuf {
+pub(crate) fn scratch_dir() -> PathBuf {
     let dir = Path::new(env!("CARGO_MANIFEST_DIR"))
+        // Pop "fuzzing".
+        .join("..")
+        // Pop "crates".
+        .join("..")
         .join("target")
         .join("scratch");