Fancy non-whitespace indentation

[e-matteson]

Now indentation can be an arbitrary string instead of just a number of spaces. This will allow comment nodes to place comment syntax in front of all of the lines they contain. We also now store doc node ids and marks for each individual level of indentation, which will allow for better cursor region highlighting.

[justinpombrio]

Reviewed. See the changes I pushed. And three suggestions:

• The Oracle's complicated append method isn't needed. It can just have append_text and append_newline, and they'll both be much simpler.
• struct Line { segments: Vec<Segment> } should probably just be struct Line(Vec<Segment>)?
• Block::new() should take an &IndentNode for effiency. Maybe? Though it isn't cloning, maybe it's already walking the Rcs without modifying their counts?

Each of these suggestions has a TODO, that should be deleted when it's done or wontfix-ed.