refactor: singleton redesign

[LHerskind]

Singletons currently have get_note and replace functions that both emit nullifiers to ensure that the old value is "up to date". This means that updating the note such that the value at t' is dependent on the value at t is quite a mess.

A description and pointers to where this is encountered can be seen in #3689.

[sirasistant]

One option is to make replace like this:

 pub fn replace<Env>(
        self,
        new_note_builder: fn[Env](old_note: Note) -> Note, // closure
        broadcast: bool,
    ) -> Note {

So it can still be safe and be used like

let new_note = singleton.replace(|old_note| {
  assert(old_note.foo == 27);
  MyNewNote::new(old_note.whatever)
}, true);

[LHerskind]

Referring to #3123 for completeness

[LHerskind]

This have been covered through the ordering and squashing work