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.

crates/fuzzing/src/generators.rs

@@ -11,6 +11,8 @@
 #[cfg(feature = "binaryen")]
 pub mod api;
 
+pub mod table_ops;
+
 use arbitrary::{Arbitrary, Unstructured};
 
 /// A Wasm test case generator that is powered by Binaryen's `wasm-opt -ttf`.