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/rule/matcher_vectree/vec_match.rs

@@ -8,6 +8,7 @@ use crate::rule::matcher::RuleExpr;
 use crate::rule::state::{State, StateEntry};
 use crate::utils::unwrap_or;
 
+#[must_use]
 pub fn vec_match<'a>(
   matcher: &VecMatcher,
   seq: &'a [RuleExpr],