additional list functions (range, map, filter, reduce, flatten) (#74)

also make the VM cope with reentrancy

crates/haku/src/value.rs

@@ -47,6 +47,13 @@ impl Value {
             _ => None,
         }
     }
+
+    pub fn to_ref(&self) -> Option<RefId> {
+        match self {
+            Self::Ref(v) => Some(*v),
+            _ => None,
+        }
+    }
 }
 
 impl From<()> for Value {