Rename Buffer.AddAt to Insert.

When editing text, you insert and delete, you don't add and remove.
As evidence, look down at your keyboard. Find the insert and delete keys.
Now look again and find the add and remove keys.
Also, insert and delete have the same number of letters,
but remove is twice as long as add.

I also excluded the At suffix.
ReadAt implements the io.ReaderAt interface.
There are no InserterAt or DeleterAt interfaces.