refactor WASM memory data passing with strict provenance

[tzemanovic]

In host-guest WASM FFI, we're doing many unsafe memory pointer conversions. This could be done better with strict provenance. We can use polyfill https://docs.rs/sptr/latest/sptr before this becomes available in stable std. For background: https://gankra.github.io/blah/tower-of-weakenings/

[cwgoes]

@tzemanovic do you know the status of this?

[tzemanovic]

to be done