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.

internal/assetstore/map.go

@@ -47,7 +47,7 @@ func (a *AssetStore) Map(name string) (*Map, error) {
 	}
 
 	m := &Map{
-		Rect: raw.Rect(),
+		Rect:   raw.Rect(),
 		assets: a,
 		raw:    raw,
 		set:    set,
@@ -74,8 +74,8 @@ func (m *Map) LoadSprites() error {
 }
 
 // FIXME: get rid of this
-func (m *Map) Cell(x, y, z int) maps.Cell {
-	return m.raw.Cells.At(x, y, z)
+func (m *Map) Cell(x, y, z int) *maps.Cell {
+	return m.raw.At(x, y, z)
 }
 
 // SpritesForCell returns the sprites needed to correctly render this cell.