Segfault dropping `Stream` if `Context` is already dropped

pw_stream_destroy segfaults if the Context was already dropped. After some other memory errors like in the call to libspa::hook::remove destroying the StreamListener. Valgrind reports no memory errors if the Context is dropped last.

I guess Context needs to be ref-counted, and CoreInner needs to contain a reference to it? Stream already has a reference to Core, but I'm not sure that's doing anything since CoreInner just contains a pointer and doesn't seem to implement Drop. Looks like a call to pw_core_destroy is missing.