Rename `get_history` to `history_mut`
Rename `get_history_const` to `history`
Replace `set_completer` by `set_helper`

And reintroduce `set_completer`.

We must redraw the line with the original prompt
when the command used to validate/exit the loop is a movement.

Reuse undo::Changeset to retrieve last inserted text.
Introduce Cmd::Replace(Movement, Option<String>).
Refactor Cmd::Kill handling.

At least to avoid the confusion with State.

thread 'main' panicked at 'already borrowed: BorrowMutError'

Ensure 0 previous rows are removed when calling refresh on a freshly
created State. Before, 0+ rows were removed on an initial State
refresh. If the prompt had a newline in it, a positive number of rows
were removed, clearing past output in stdout.

Fix #109