Project var state

[AlexeyRaga]

1. Allow projecting state

// Project individual fields from the Execution output
override _.Update(state, input, outputVar) =
    { Name = output |> Var.map _.Name
      Age  = output |> Var.map _.Age }

2. Report state better

When the state contains Vars:

public record CounterState
{
    public required Var<int> CurrentCount { get; init; }
}

Resolved values are shown in the error report:

Counterexamples:
  Initial state: CounterState { CurrentCount = 0 }
  + Reset 
  Set 21
  AddRandom 
  Failed at state: CounterState { CurrentCount = 21 }