Actor-served files: /actor/<id>/fs/

File operations under /actor/<id>/fs/ become messages from `fs` to <id>
(fs_read, fs_write, fs_mkdir, fs_remove, fs_rename) with a 10 s timeout.
vfs.rs wraps the wasi:filesystem implementation of wasmtime-wasi.

cli/src/world.rs

@@ -22,8 +22,9 @@ use serde::{Deserialize, Serialize};
 /// Name prefix of the host's temporary files and folders. Copies skip them.
 pub const TMP_PREFIX: &str = ".sausage-tmp-";
 
-/// IDs of the actors that the host implements natively.
-pub const NATIVE: &[&str] = &["broker", "console", "cargo"];
+/// IDs of the actors that the host implements natively. `fs` sends the
+/// requests for actor-served files (`vfs.rs`).
+pub const NATIVE: &[&str] = &["broker", "console", "cargo", "fs"];
 
 #[derive(Clone)]
 pub struct World {