Use WAMR correctly

[r1ru]

wasmos exports some functions for message passing to WASM.

https://github.com/r1ru/wasmos/blob/main/kernel/wasmvm.c#L102-L108

According to the documentation, data should be serialised and deserialised when passing structured data, and the type "$" should not be used as a pointer to a message. Furthermore, the platform API layer is implemented in platform.c to port WAMR to wasmos, but it is mostly a missing implementation. Thus, for example, it is currently not possible to display WAMR error messages because snprintf is not implemented.