Better clifmt/text system

[nic-hartley]

Text should be a blob of text with potentially several different formats within it, rather than a single string with one format. Then what's currently Text could probably be Segment. That would allow for convenience functions like checking equivalence, merging/minimizing, iterating over Cells, etc.

• Rename Text to Segment, text1! to segment!, etc.
• Add a new struct Text which contains a Vec<Segment>, and tweak text! to return that
• Add iterator for Cells
• Implement PartialEq to compare cell by cell
• Tweak tests (e.g. text_input, but probably more) to simplify with the new text system
• Editing formatting for ranges even if they don't line up with existing Segments