Switch from encoding/binary to struc

It's not perfect, but struc can deserialize the whole thing into one
struct while encoding/binary can't. It's nice to have that.

cmd/view-minimap/main.go

@@ -181,8 +181,8 @@ func (e *env) Draw(screen *ebiten.Image) error {
 
 	for y := int(rect.Min.Y); y < int(rect.Max.Y); y++ {
 		for x := int(rect.Min.X); x < int(rect.Max.X); x++ {
-			cell := gameMap.Cells.At(x, y, int(e.state.zIdx))
-			imd.Set(x, y, makeColour(&cell, e.state.cellIdx))
+			cell := gameMap.At(x, y, int(e.state.zIdx))
+			imd.Set(x, y, makeColour(cell, e.state.cellIdx))
 		}
 	}