extract minimal room size as constant

src/constants.rs

@@ -12,6 +12,12 @@ pub const ROOM_WIDTH: usize = 9;
 /// the height of a room in the grid of rooms (number of characters)
 pub const ROOM_HEIGHT: usize = 6;
 
+// the minmal width of a room
+pub const ROOM_MIN_WIDTH: usize = 4;
+
+// the minmal height of a room
+pub const ROOM_MIN_HEIGHT: usize = 4;
+
 /// How many levels does the dungeon have?
 pub const LEVELS: usize = 2;