Added directfs

Added a very rudimentary file I/O system suitable for experimenting
with the language further. A better one will be designed when we have
sensible error management.

src/systems/io/mod.rs

@@ -9,3 +9,4 @@ mod service;
 
 // pub use facade::{io_system, IOStream, IOSystem};
 pub use service::{Service, Stream, StreamTable};
+pub use instances::{wrap_io_error, Source, Sink};