refactor(Notes): Make get_notes return all notes at the beginning of the array

[nventuro]

get_notes returns Option<Note>[], with the Some values in the same order as they were returned from the PXE. This means that if e.g. we were to call get_notes with a limit of 1 and merge #4988 (so that the function returns one and only one note), we wouldn't know where in the array the note is and would need to manually search for it. Similarly, processing all notes requires looking at the entire array, potentially keeping track of e.g. how many notes we've seen.

I worry that this makes it hard to properly constrain applications that read notes. While a cooperative PXE would return all notes at the beginning, a malicious one might choose not to and exploit applications that incorrectly assume e.g. that the first note is valid.

I propose we make get_notes constrain the returned array so that all notes are placed at the beginning of the array. We can easily do this by having a second array that we copy into with read and write indices, or even do it in place with careful swapping of None and Some values.

[nventuro]

While the single return value case is clear (and a common use case), the line is a bit blurred when working with multiple notes, since lack of break means we'll always iterate over all notes anyway, even after finding the first None case (which would indicate all further options are also None).

It is unclear to me how useful this feature would be in this case, though of course break could always be added. But other features such as filter or if let would similarly make looping simpler without requiring any constraints about ordering:

let mut count = 0;

// filter
for some_note in opt_notes.filter(|opt| opt.is_some()):
    count += some_note.unwrap_unchecked().value

// if let
for opt_note in opt_notes:
  if let Some(note) = opt_note:
    count += note.value