Bring back Module::deserialize (#2858)

* Bring back `Module::deserialize`

I thought I was being clever suggesting that `Module::deserialize` was
removed from #2791 by funneling all module constructors into
`Module::new`. As our studious fuzzers have found, though, this means
that `Module::new` is not safe currently to pass arbitrary user-defined
input into. Now one might pretty reasonable expect to be able to do
that, however, being a WebAssembly engine and all. This PR as a result
separates the `deserialize` part of `Module::new` back into
`Module::deserialize`.

This means that binary blobs created with `Module::serialize` and
`Engine::precompile_module` will need to be passed to
`Module::deserialize` to "rehydrate" them back into a `Module`. This
restores the property that it should be safe to pass arbitrary input to
`Module::new` since it's always expected to be a wasm module. This also
means that fuzzing will no longer attempt to fuzz `Module::deserialize`
which isn't something we want to do anyway.

* Fix an example

* Mark `Module::deserialize` as `unsafe`

tests/all/module_linking.rs

@@ -39,7 +39,9 @@ fn compile() -> Result<()> {
     assert_eq!(m.imports().len(), 0);
     assert_eq!(m.exports().len(), 0);
     let bytes = m.serialize()?;
-    Module::new(&engine, &bytes)?;
+    unsafe {
+        Module::deserialize(&engine, &bytes)?;
+    }
     assert_eq!(m.imports().len(), 0);
     assert_eq!(m.exports().len(), 0);
     Ok(())