Correct some oddities

* import only the `generateHexString` function
* declare the `typdef` on one common JSDoc comment after the imports
* remove unnecessary helper function

src/shared/test/random-test.js

-import * as random from '../random';
+import { generateHexString } from '../random';
 
 describe('sidebar.util.random', () => {
   describe('#generateHexString', () => {
     [2, 4, 8, 16].forEach(len => {
       it(`returns a ${len} digit hex string`, () => {
         const re = new RegExp(`^[0-9a-fA-F]{${len}}$`);
-        const str = random.generateHexString(len);
+        const str = generateHexString(len);
         assert.isTrue(re.test(str));
       });
     });

src/sidebar/services/annotations.js

-/** @typedef {import('../../types/api').Annotation} Annotation */
-/** @typedef {import('../../types/annotator').AnnotationData} AnnotationData */
-
 import { generateHexString } from '../../shared/random';
 import * as metadata from '../helpers/annotation-metadata';
 import {
@@ -9,6 +6,11 @@ import {
   sharedPermissions,
 } from '../helpers/permissions';
 
+/**
+ * @typedef {import('../../types/api').Annotation} Annotation
+ * @typedef {import('../../types/annotator').AnnotationData} AnnotationData
+ */
+
 /**
  * A service for creating, updating and persisting annotations both in the
  * local store and on the backend via the API.

src/sidebar/util/postmessage-json-rpc.js

 import { generateHexString } from '../../shared/random';
 
-/** Generate a random ID to associate RPC requests and responses. */
-function generateId() {
-  return generateHexString(10);
-}
-
 /**
  * Return a Promise that rejects with an error after `delay` ms.
  */
@@ -33,7 +28,7 @@ export function call(
   params = [],
   timeout = 2000,
   window_ = window,
-  id = generateId()
+  id = generateHexString(10)
 ) {
   // Send RPC request.
   const request = {