refactor the whole damn thing

src/Struct/Type.js

@@ -0,0 +1,26 @@
+import Item from './Item'
+
+export default class Type extends Item {
+  constructor () {
+    super()
+    this._map = new Map()
+    this._start = null
+  }
+  _delete (y) {
+    super._delete(y)
+    // delete map types
+    for (let value of this._map.values()) {
+      if (value instanceof Item && !value._deleted) {
+        value._delete()
+      }
+    }
+    // delete array types
+    let t = this._start
+    while (t !== null) {
+      if (!t._deleted) {
+        t._delete()
+      }
+      t = t._right
+    }
+  }
+}