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/Cargo.toml

@@ -57,6 +57,12 @@ path = "fuzz_targets/spectests.rs"
 test = false
 doc = false
 
+[[bin]]
+name = "table_ops"
+path = "fuzz_targets/table_ops.rs"
+test = false
+doc = false
+
 [[bin]]
 name = "peepmatic_simple_automata"
 path = "fuzz_targets/peepmatic_simple_automata.rs"