Simplify and improve the topline logic for the debug map
Stairs->other_side_known
Make use of other_side in debug-map < and > commands
Use before instead of useless override/super
Generalize Stairs into LevelChangers

    We'll need a Portal tile class eventually, so better to start
    supporting it now :)
Fix Debug Map to check for LevelChangers rather than Stairs
When we traverse a level-changer in the map, set x and y too
Avoid useless repeated calls to $self->{tiles}->[$y]
Remove busted PREFIX=~/.local instructions. mst++
Merge remote-tracking branch 'sartak/master'