Move stdlib into its own crate

stdlib/src/set.rs

@@ -0,0 +1,42 @@
+use im_rc::hashset::HashSet;
+use std::hash::{Hash};
+use im_rc::vector::Vector;
+
+/// A persistent Set which records insertion order and iterates in that order.
+pub struct Set<K> {
+    store: HashSet<K>,
+    order: Vector<K>
+}
+
+impl<K> Set<K>
+where K : std::hash::Hash
+{
+    pub fn is_empty(self) -> bool {
+        self.store.is_empty()
+    }
+
+    pub fn insert<B>(self, elem: K) -> Set<B>
+    {
+        let mut new_set: Set<K> = self.clone();
+
+        new_set.store.insert(elem);
+        new_set.order.insert(elem);
+
+        new_set
+    }
+
+    pub fn map<F, B>(self, transform: F) -> Set<B>
+    where F: Fn(K) -> B,
+        B: Hash
+    {
+        let mut new_set: Set<B> = Set::new();
+
+        for elem in self.order.iter() {
+            if self.store.contains(elem) {
+                new_set.insert(transform(elem))
+            }
+        }
+
+        new_set
+    }
+}