Previously the isUnit function was falsely returning `True` for
positions that were one tile off in *either* direction from the
character, when it should've been *both*. Oops.
When gormlaks see the character, they step towards them and attack
dealing 1 damage when adjacent. Characters have hitpoints now, displayed
at the bottom of the game screen, and when the game is over they die.
Add a Brain class, which determines for an entity the set of moves it
makes every step of the game, and begin to implement that for gormlaks.
The idea here is that every step of the game, a gormlak will move
towards the furthest-away wall it can see.
Add a Door entity and an Open command, which necessitated supporting the
direction prompt. Currently nothing actually puts doors on the map,
which puts a slight damper on actually testing this out.
As the character walks around the map, progressively reveal the entities
on the map to them, using an algorithm based on well known
circle-rasterizing and line-rasterizing algorithms to calculate lines of
sight that are potentially obscured by walls.
Add support for converting generated levels to walls, and merge one into
the entity map at the beginning of the game.
There's nothing here that guarantees the character ends up *inside* the
level though (they almost always don't) so that'll have to be slotted
into the level generation process.
