message: Simplify a bit the wasi integration
diff: use crate::utils::{parse_envvar, parse_mapdir};
  use anyhow::{Context, Result};
  use std::path::PathBuf;
  -use wasmer::{Function, Instance, Memory, Module};
  +use wasmer::{Instance, Module};
  use wasmer_wasi::{get_wasi_version, WasiState, WasiVersion};
  use structopt::StructOpt;
  @@ -56,14 +56,11 @@ impl Wasi {
  let mut wasi_env = wasi_state_builder.finalize()?;
  let import_object = wasi_env.import_object(&module)?;
  -
  let instance = Instance::new(&module, &import_object)?;
  - let memory: &Memory = instance.exports.get("memory")?;
  - wasi_env.set_memory(memory);
  -
  - let start: &Function = instance.exports.get("_start")?;
  + wasi_env.set_memory(instance.exports.get_memory("memory")?);
  + let start = instance.exports.get_function("_start")?;
  start
  .call(&[])
  .with_context(|| "failed to run WASI `_start` function")?;
  