简化语法

src/base64.js

@@ -2,12 +2,8 @@ import { encode, decode } from './helper.js'
 
 // string 转 binary
 export function str2bin(str) {
-  let bin = ''
   let u8 = encode(str) // 转成Uint8Array
-  for (let i = 0; i < u8.length; i++) {
-    bin += String.fromCharCode(u8[i])
-  }
-  return bin
+  return String.fromCharCode.apply(null, u8)
 }
 
 export function bin2str(b) {