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/interpreter/apply.rs

@@ -48,6 +48,7 @@ fn map_at<E>(
 /// Replace the [Clause::LambdaArg] placeholders at the ends of the [PathSet]
 /// with the value in the body. Note that a path may point to multiple
 /// placeholders.
+#[must_use]
 fn substitute(paths: &PathSet, value: Clause, body: ExprInst) -> ExprInst {
   let PathSet { steps, next } = paths;
   unwrap_always(map_at(steps, body, &mut |checkpoint| -> Always<Clause> {

src/interpreter/handler.rs

@@ -21,6 +21,7 @@ pub struct HandlerTable<'a> {
 }
 impl<'a> HandlerTable<'a> {
   /// Create a new [HandlerTable]
+  #[must_use]
   pub fn new() -> Self { Self { handlers: HashMap::new() } }
 
   /// Add a handler function to interpret a type of atom and decide what happens
@@ -46,6 +47,7 @@ impl<'a> HandlerTable<'a> {
   }
 
   /// Combine two non-overlapping handler sets
+  #[must_use]
   pub fn combine(mut self, other: Self) -> Self {
     for (key, value) in other.handlers {
       let prev = self.handlers.insert(key, value);