message: fix(wasi) Replace an `unwrap` by an `expect`.
  Calling `self.memory()` is better in that case because it unwraps
  `self.memory.get_memory()` with the `expect` method, which provides
  guidances to the user.
diff: @@ -177,7 +177,7 @@ impl WasiEnv {
  &mut self,
  _mem_index: u32,
  ) -> (&Memory, MutexGuard<WasiState>) {
  - let memory = self.memory.get_memory().unwrap();
  + let memory = self.memory();
  let state = self.state.lock().unwrap();
  (memory, state)
  }
  