Saving WIP on grand refactor
I prefer to keep the signature close to the definition
Use existential types for heterogenous monster list
Load level 1 in startGame
Use 'Z' for Zombie
Add `operational` to dependencies
Refactor engine to use Operational

 * Remove lens for now as it's currently not used
 * All actors are implemented as continuation based programs
 * The Actor API is defined as ActorOp operations which allow actors to
   perform actions and query the game state.
Make walls more interesting with Unicode
Z is for Zombie
Don't fetch position unless we need it
Formatting
Improve some consistency
add _very_ basic zombie AI
fix some infinite looping when crashing into walls
only redraw before we need input
Only draw before we begin processing the next tick

Collect processed actors in xs' which we reverse and evaluate for the
next tick cycle. When xs' is empty we know we're at the start of a new
tick cycle and it makes sense to draw the result of the last evaluated
tick.
Remove UTF8 again

My current font didn't know how to display the Unicode wall glyph :p
Only export `treasure`
let actors work until explicitly yielding
fix bug in fillVoid