text-size

• add TextRange::ordering method

• the carate is renamed to text-size from text_unit

Transition table:

• TextUnit::of_char(c) ⟹ TextSize::of(c)
• TextUnit::of_str(s) ⟹ TextSize::of(s)
• TextUnit::from_usize(size) ⟹ TextSize::try_from(size).unwrap_or_else(|| panic!(_))
• unit.to_usize() ⟹ usize::from(size)
• TextRange::from_to(from, to) ⟹ TextRange::new(from, to)
• TextRange::offset_len(offset, size) ⟹ TextRange::from_len(offset, size)
• range.start() ⟹ range.start()
• range.end() ⟹ range.end()
• range.len() ⟹ range.len()
• range.is_empty() ⟹ range.is_empty()
• a.is_subrange(b) ⟹ b.contains_range(a)
• a.intersection(b) ⟹ a.intersect(b)
• a.extend_to(b) ⟹ a.cover(b)
• range.contains(offset) ⟹ range.contains(point)
• range.contains_inclusive(offset) ⟹ range.contains_inclusive(point)