wasmtime: Add lots of logging for externrefs and table_ops fuzz target (#4583)

I essentially add these same logs back in every time I'm debugging something
related to this fuzz target or `externref`s in general. Probably like 5 times
I've added roughly these logs. We should just make them available whenever we
need them via `RUST_LOG=wasmtime_runtime=trace`.

This also changes a couple `if let`s to `unwrap`s that are now infallible after

cranelift/codegen/src/machinst/buffer.rs

@@ -1338,6 +1338,7 @@ impl<I: VCodeInst> MachBuffer<I> {
                 (start_offset, end_offset)
             }
         };
+        trace!("Adding stack map for offsets {start:#x}..{end:#x}");
         self.stack_maps.push(MachStackMap {
             offset: start,
             offset_end: end,