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/pipeline/dealias/walk_with_links.rs

@@ -12,6 +12,7 @@ pub enum Target<'a, N: NameLike> {
   Leaf(&'a ProjectItem<N>),
 }
 
+#[must_use = "this is the sole product of this function"]
 pub struct WalkReport<'a, N: NameLike> {
   pub target: Target<'a, N>,
   pub abs_path: VName,