fuzz: Add a fuzz target for table.{get,set} operations

This new fuzz target exercises sequences of `table.get`s, `table.set`s, and
GCs.

It already found a couple bugs:

* Some leaks due to ref count cycles between stores and host-defined functions
  closing over those stores.

* If there are no live references for a PC, Cranelift can avoid emiting an
  associated stack map. This was running afoul of a debug assertion.

fuzz/fuzz_targets/table_ops.rs

@@ -0,0 +1,9 @@
+#![no_main]
+
+use libfuzzer_sys::fuzz_target;
+use wasmtime_fuzzing::generators::{table_ops::TableOps, Config};
+
+fuzz_target!(|pair: (Config, TableOps)| {
+    let (config, ops) = pair;
+    wasmtime_fuzzing::oracles::table_ops(config, ops);
+});