`Recorder::clear` can create permanent entries

[Treeniks]

It seems inserts between a previous commit and a call to clear don't get managed properly, as they appear permanently within the container after an undo.

The following code:

use std::collections::BTreeMap;
use undoredo::Recorder;
use undoredo::UndoRedo;
use undoredo::aliases::BTreeMapDelta;

let mut recorder: Recorder<BTreeMap<usize, char>> = Recorder::new(BTreeMap::new());
let mut undoredo: UndoRedo<BTreeMapDelta<usize, char>> = UndoRedo::new();

recorder.insert(1, 'X');
undoredo.commit(&mut recorder);
println!("a: {:?}", recorder.container());

recorder.insert(2, 'Y');
recorder.clear();
undoredo.commit(&mut recorder);
println!("b: {:?}", recorder.container());

undoredo.undo(&mut recorder);
println!("c: {:?}", recorder.container());

undoredo.redo(&mut recorder);
println!("d: {:?}", recorder.container());

produces this output:

a: {1: 'X'}
b: {}
c: {1: 'X', 2: 'Y'}
d: {2: 'Y'}

I would expect a and c to be equivalent, as well as b and d. In particular, the 2: 'Y' entry should disappear permanently as it was never actually committed. Yet it stays even after a call to redo.