如何在Golang中使用AES256-GCM加密文件？

AES256-GCM could be implemented in go as https://gist.github.com/cannium/c167a19030f2a3c6adbb5a5174bea3ff

However, Seal method of interface cipher.AEAD has signature:

Seal(dst, nonce, plaintext, additionalData []byte) []byte

So for very large files, one must read all file contents into memory, which is unacceptable.

A possible way is to implement Reader/Writer interfaces on Seal and Open, but shouldn't that be solved by those block cipher "modes" of AEAD? So I wonder if this is a design mistake of golang cipher lib, or I missed something important with GCM?