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/parse/context.rs

@@ -8,8 +8,11 @@ use crate::{Tok, VName};
 /// Hiding type parameters in associated types allows for simpler
 /// parser definitions
 pub trait Context: Clone {
+  #[must_use]
   fn ops(&self) -> &[Tok<String>];
+  #[must_use]
   fn file(&self) -> Rc<VName>;
+  #[must_use]
   fn interner(&self) -> &Interner;
 }