refactor: role stores (#2478)

* refactor: reduced duplication in role stores

* fix docs

* fix: typo

* most of requested changes

* rest of changes

* ACTUAL rest of changes

* docs

* doooocs

* reason

src/util/Util.js

@@ -1,4 +1,5 @@
 const snekfetch = require('snekfetch');
+const Collection = require('./Collection');
 const { Colors, DefaultOptions, Endpoints } = require('./Constants');
 const { Error: DiscordError, RangeError, TypeError } = require('../errors');
 const has = (o, k) => Object.prototype.hasOwnProperty.call(o, k);
@@ -413,6 +414,33 @@ class Util {
       setTimeout(resolve, ms);
     });
   }
+
+  /**
+   * Adds methods from collections and maps onto the provided store
+   * @param {DataStore} store The store to mixin
+   * @param {string[]} ignored The properties to ignore
+   * @private
+   */
+  /* eslint-disable func-names */
+  static mixin(store, ignored) {
+    Object.getOwnPropertyNames(Collection.prototype)
+      .concat(Object.getOwnPropertyNames(Map.prototype)).forEach(prop => {
+        if (ignored.includes(prop)) return;
+        if (prop === 'size') {
+          Object.defineProperty(store.prototype, prop, {
+            get: function() {
+              return this._filtered[prop];
+            },
+          });
+          return;
+        }
+        const func = Collection.prototype[prop];
+        if (prop === 'constructor' || typeof func !== 'function') return;
+        store.prototype[prop] = function(...args) {
+          return func.apply(this._filtered, ...args);
+        };
+      });
+  }
 }
 
 module.exports = Util;