perf: improve omit performance (#548)

Author: linxianxi

src/omit.ts

@@ -2,7 +2,7 @@ export default function omit<T extends object, K extends keyof T>(
   obj: T,
   fields: K[] | readonly K[],
 ): Omit<T, K> {
-  const clone = { ...obj };
+  const clone = Object.assign({}, obj);
 
   if (Array.isArray(fields)) {
     fields.forEach(key => {