Option match invert onNone and onSome

[SandroMaglione]

⚠️: Breaking change

This PR inverts the onNone and onSome in the match method of the Option type.

As mentioned in #52, having the Some case first and the None (error) case second was not intuitive. This changes also aligns Option with Either, where the error (Left) is expected first and the success (Right) second.

Furthermore, this PR also adds the fold method to Option, equivalent to match (in order to align Option to Either).

/// Previously
final match = option.match(
  (a) => print('Some($a)'),
  () => print('None'), // <- `None` second 👎 
);

/// New
final match = option.match(
  () => print('None'), // <- `None` first 👍
  (a) => print('Some($a)'),
);
// or
final match = option.fold(
  () => print('None'),
  (a) => print('Some($a)'),
);