Limit size of linear memory in VM

[webmaster128]

The heap data of a smart contract execution is stored in a Wasm linear memory. Only zero or one linear memories can exist in the current Wasm standard. The maximum size of a linear memory is 4GB because it is addressed by uint32 offsets in 1 byte steps.

Right now, we ensure the upper memory limit is unset by the contract:

fn check_wasm_memories(module: &Module) -> VmResult<()> {
    // ...

    if limits.maximum() != None {
        return Err(VmError::static_validation_err(
            "Wasm contract memory's maximum must be unset. The host will set it for you.",
        ));
    }
    Ok(())
}

If we wanted we could implement the limit the following way at any time: take a Wasm blob, deserialize, set the maximum of the first linear memory, re-serialize, compile. This is not very elegant and has the disadvantage that the memory limit is stored as part of the compiled modules. So it cannot be adjusted without invalidating cached modules.

A proper solution is discussed here, which is what we wait for right now: wasmerio/wasmer#1360

[webmaster128]

Depends on #503

[webmaster128]

This probably becomes very easy with Wasmer 1.0 (follow wasmerio/wasmer#1730 for updates)