editor/view: expose more efficient method for making change-style edits.

[eaburns]

View.Do is expensive. It allows any edit to be made, even edits that changes the tracked marks. In order properly track the marks, View.Do does a complete refresh of the text and marks. Most edits made by T, are simple change-style edits that cannot affect marks. We should add a special method to efficiently make change-style edits without blocking on a complete refresh: View.Change(edit.Address, str string).