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/representations/postmacro_to_interpreted.rs

@@ -5,6 +5,7 @@ use super::path_set::PathSet;
 use super::{interpreted, postmacro};
 use crate::utils::Side;
 
+#[must_use]
 fn collect_paths_expr_rec(
   expr: &postmacro::Expr,
   depth: usize,