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.

fuzz/fuzz_targets/api_calls.rs

@@ -0,0 +1,27 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use std::sync::Once;
+use wasmtime_fuzzing::{generators::api::ApiCalls, oracles};
+
+fuzz_target!(|api: ApiCalls| {
+    static INIT_LOGGING: Once = Once::new();
+    INIT_LOGGING.call_once(|| env_logger::init());
+
+    log::debug!(
+        "If this fuzz test fails, here is a regression tests:
+```
+#[test]
+fn my_regression_test() {{
+    use wasmtime_fuzzing::generators::{{
+        api::{{ApiCall::*, ApiCalls}},
+        WasmOptTtf,
+    }};
+    wasmtime_fuzzing::oracles::make_api_calls({:#?});
+}}
+```",
+        api
+    );
+
+    oracles::make_api_calls(api);
+});