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/scenario/manage.go

@@ -9,7 +9,7 @@ type CellPoint struct {
 	Z int
 }
 
-func (s *Scenario) CellAtCursor() (maps.Cell, CellPoint) {
+func (s *Scenario) CellAtCursor() (*maps.Cell, CellPoint) {
 	cell := s.area.Cell(int(s.selectedCell.X), int(s.selectedCell.Y), 0)
 	return cell, CellPoint{IsoPt: s.selectedCell, Z: 0}
 }