Implement entity creation

Closes #6 (closed).

packages/ra-cubicweb/src/utils/__tests__/deduplicate.ts

+import { deduplicate } from "../deduplicate";
+
+describe("deduplicate", () => {
+  it('returns ["foo"] for ["foo", "foo"]', () => {
+    const deduplicated = deduplicate(["foo", "foo"]);
+
+    expect(deduplicated).toEqual(["foo"]);
+  });
+});