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/facade/environment.rs

@@ -23,9 +23,11 @@ pub struct Environment<'a> {
 }
 impl<'a> Environment<'a> {
   /// Initialize a new environment
+  #[must_use]
   pub fn new(i: &'a Interner) -> Self { Self { i, systems: Vec::new() } }
 
   /// Register a new system in the environment
+  #[must_use]
   pub fn add_system<'b: 'a>(mut self, is: impl IntoSystem<'b> + 'b) -> Self {
     self.systems.push(Box::new(is).into_system(self.i));
     self