Generalize action building

[rhalbersma]

• store visited squares and jump order (suitable for GUIs that do jump path animation)
• counting container (incrementing a counter, rather than building an action) when keeping duplicates
  • for trivial jump precedence
  • for non-trivial jump precedence
• successor container (building the successor state, rather than an action)
• make an action_view (inspired by std::string_view) that can adopt either
  • an action
  • a state and an action
  • two states
• action_view should support
  • jump precedence comparisons
  • make on states
  • undo on states