Implement Undo

[ndd7xv]

Background

heh currently has no way to undo a deleted or back-spaced byte. It would also be convenient for users to "un-type" things are go back to a position the cursor was previously located at.

Task

I'm thinking to implement something basic, we could have a Vector of Actions, which would be an enumeration of the different actions a user will be able to undo. Hitting ctrl+z would pop actions from the vector and undo that command, while doing an action would push it to the vector.