Factor LineV (read-only) from Line (read-write).
Mark is property of buffer, not view.

Also moved bound() into Buffer, since it makes more sense there.
Use SPACE to talk about the space key.
Support for manipulating regions.

A region is the text between two locs, which can encompass fragments of a
starting and ending line, and zero or more full lines in between. With this, we
can start on killing and yanking.