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/lib.rs

@@ -9,6 +9,7 @@ pub mod cargo_actor;
 pub mod console;
 pub mod runtime;
 pub mod supervisor;
+pub mod vfs;
 pub mod world;
 
 use std::path::Path;