docs(ArrayBuffer): optimize ab2str()

docs/arraybuffer.md

@@ -452,9 +452,19 @@ Float64Array.BYTES_PER_ELEMENT // 8
 ```javascript
 // ArrayBuffer转为字符串，参数为ArrayBuffer对象
 function ab2str(buf) {
+  if (buf && buf.byteLength < 128000) {	// limit max length avoid overhead
     return String.fromCharCode.apply(null, new Uint16Array(buf));
   }
 
+  const len =  bufView.byteLength;
+  const bstr = new Array(len);
+  const bufView = new Uint16Array(buf);
+  for (let i = 0; i < len; i++) {
+    bstr[i] = String.fromCharCode.call(null, bufView[i]);
+  }
+  return bstr.join('');
+}
+
 // 字符串转为ArrayBuffer对象，参数为字符串
 function str2ab(str) {
   const buf = new ArrayBuffer(str.length * 2); // 每个字符占用2个字节