Rivers and Bridges map refactor (#103)

* Separated out rivers and bridges into its own file with three map variants

* fixing tags

* removed extra 4 starting positions from the medium map since it only supports 8 players

* update GetUnoccupiedPoints to consider hazards with a flag

* use new utility method to fine unoccupied points and enforce map sizes

* changed up casting to make IsAllowable() more usable

maps/game_map.go

@@ -44,6 +44,20 @@ func (d sizes) IsUnlimited() bool {
 	return len(d) == 1 && d[0].Width == 0
 }
 
+func (d sizes) IsAllowable(Width int, Height int) bool {
+	if d.IsUnlimited() {
+		return true
+	}
+
+	for _, size := range d {
+		if size.Width == uint(Width) && size.Height == uint(Height) {
+			return true
+		}
+	}
+
+	return false
+}
+
 // AnySize creates sizes for a board that has no fixed sizes (supports unlimited sizes).
 func AnySize() sizes {
 	return sizes{Dimensions{Width: 0, Height: 0}}