Add infrastructure to Parley to facilitate text selection/editing

[PoignardAzur]

We want Parley to implement types and methods that Masonry and other editors will be able to use to create a text-editing widget, or to handle selection in non-editable text.

After some discussion we've settled on the following:

• A TextMovement enum representing different possible changes to a text edit's selection and content. Possible variants might be eg MoveLeft, MoveToNextWord, SelectToNextParagraph, RemovePreviousWord, etc.
• A function that maps a String, a cursor position(/span) and a TextMovement to a StringDiff and position(/span). That function essentially tells you "if you do that, here's how the text will change and where your cursor will be next".
• Later down the road, once we want to implement spans, a function that maps a span and a StringDiff to an optional span.

Possible signatures:

enum TextMovement {
  MoveLeft,
  MoveRight,
  // ...
}

struct StringDiff {
  removed_span: usize..usize,
  new_position: usize,
  added_text: String,
}

fn apply_movement(string: &str, movement: TextMovement, byte_offset: usize) -> StringDiff;

fn map_span(span: usize..usize, diff: StringDiff) -> Option<usize..usize>;

We night use Parley's Cursor type instead of byte offsets in these interfaces.