reorg, remove old music slice files

app/util/state.ts

@@ -0,0 +1,17 @@
+import { ById } from '@app/models/state'
+import merge from 'lodash.merge'
+
+export function reduceById<T extends { id: string }>(collection: T[]): ById<T> {
+  return collection.reduce((acc, value) => {
+    acc[value.id] = value
+    return acc
+  }, {} as ById<T>)
+}
+
+export function mergeById<T extends { [id: string]: unknown }>(object: T, source: T): void {
+  merge(object, source)
+}
+
+export function mapById<T>(object: ById<T>, ids: string[]): T[] {
+  return ids.map(id => object[id]).filter(a => a !== undefined)
+}