rework api

[Luro02]

Currently the api of a sub_cursor is like this

SubCursor::new() // empty SubCursor

and

let sub_cursor = SubCursor::from(Cursor::new(vec![]))
    .start(5)
    .end(100)
    .preserve(false);

Fields can only be accessed by calling get_end or get_start, which looks ugly

The new api should be

let sub_cursor = SubCursor::builder()
    .start(5)
    .end(100)
    .preserve(false)
    .build(Cursor::new(vec![1, 2, 3])?;

assert_eq!(sub_cursor.end(), 100);
assert_eq!(sub_cursor.start(), 5);

// setter
sub_cursor.set_start(value);
sub_cursor.set_end(value);

The new signature of SubCursor::new()

SubCursor::new(cursor: T, start: usize, end: usize, preserve: bool);