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/generators.rs

@@ -8,15 +8,28 @@
 //! wrapper over an external tool, such that the wrapper implements the
 //! `Arbitrary` trait for the wrapped external tool.
 
+pub mod api;
+
 use arbitrary::{Arbitrary, Unstructured};
+use std::fmt;
 
 /// A Wasm test case generator that is powered by Binaryen's `wasm-opt -ttf`.
-#[derive(Debug)]
+#[derive(Clone)]
 pub struct WasmOptTtf {
     /// The raw, encoded Wasm bytes.
     pub wasm: Vec<u8>,
 }
 
+impl fmt::Debug for WasmOptTtf {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(
+            f,
+            "WasmOptTtf {{ wasm: wat::parse_str(r###\"\n{}\n\"###).unwrap() }}",
+            wasmprinter::print_bytes(&self.wasm).expect("valid wasm should always disassemble")
+        )
+    }
+}
+
 impl Arbitrary for WasmOptTtf {
     fn arbitrary<U>(input: &mut U) -> Result<Self, U::Error>
     where