Statusline indicator showing how many lines across all selections

[mandx]

This indicator shows how many lines total are used by all selections in the current view.

• One or more selections on the same line only counts that line once
• A single cursor counts that line as well
• A selection that spans several lines will count all those lines as well.

Notes:

• The idea is, after making selections, if we run the extend_to_line_bounds command, how many lines do we get? That's the count I would like shown here.
• Maybe a zero width selection shouldn't be counted?
• Current implementation uses HashSet, maybe there's a better implementation that can use ranges only?