Enable `WasmStr`/`WasmList` with `call_concurrent`

Right now it's not possible to use these types as return values of a guest function with `call_concurrent`. The reason for this is that post-return is invoked automatically meaning that using these types would be using already-free'd memory in the guest. We should enable these types to work to unlock the possibility of zero-copy return values. This'll probably look like some sort of closure/trait passed to `call_concurrent` (ish) which is invoked once the values are received.

This is somewhat related to #12427 insofar as post-return is causing issues there.