Move stdlib into its own crate

stdlib/src/map.rs

@@ -0,0 +1,15 @@
+use im_rc::hashmap::HashMap;
+use im_rc::vector::Vector;
+use fxhash::FxHasher;
+
+/// A persistent HashMap which records insertion order and iterates in that order.
+pub struct Map<K, V> {
+    store: HashMap<K, V, BuildHasherDefault<FxHasher>>;
+    order: Vector<K>
+}
+
+impl<K, V> Map<K, V> {
+    pub fn is_empty(self) -> bool {
+        self.store.is_empty()
+    }
+}