7540 - baremetal: cursor

I spent over a week agonizing over this.

* I had to come to terms with the fact that I don't really know how to
make pixel graphics testable. ASCII art on a pixel by pixel basis feels
inhuman. Just focus on text mode for now.

* I had to set aside the problem of non-English family languages.
Languages like Arabic that can stack complex graphemes together likely
can't be fixed-width. But I don't know enough at the moment to solve for
them.

* I spent a while trying to juggle two cursors, one invisible output
cursor for tracking the most recent print, and one visible one that's
just a hint to the user of what the next keystroke will do. But it looks
like I can fold both into a single cursor. Drawing things updates the
location of the cursor but doesn't display it. Explicitly moving the
cursor displays it, but future drawing can overwrite the cursor.

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