Updated map to be more like set.

https://stackoverflow.com/questions/34018908/golang-why-dont-we-have-a-set-datastructure
See map[KEY]struct{} section.  This uses 0 bytes instead of
storing bool.

starfield.go

@@ -21,7 +21,7 @@ type StarField struct {
 	RNG *rand.Rand
 	MX  int
 	MY  int
-	Sky map[StarPos]bool
+	Sky map[StarPos]struct{}
 }
 
 func (s *StarField) make_pos() StarPos {
@@ -37,7 +37,7 @@ func (s *StarField) make_pos() StarPos {
 func (s *StarField) Regenerate() {
 	s.MX = door.Width
 	s.MY = door.Height
-	s.Sky = make(map[StarPos]bool)
+	s.Sky = make(map[StarPos]struct{})
 
 	s.RNG = rand.New(mt19937.New())
 	s.RNG.Seed(time.Now().UnixNano())
@@ -47,7 +47,7 @@ func (s *StarField) Regenerate() {
 		pos := s.make_pos()
 		pos.Symbol = i % 2
 		pos.Color = i%5 < 2
-		s.Sky[pos] = true
+		s.Sky[pos] = struct{}{}
 	}
 }