polish: add ClaimsChanged method to power shim

chain/actors/builtin/power/power.go

@@ -56,6 +56,7 @@ type State interface {
 	MinerNominalPowerMeetsConsensusMinimum(address.Address) (bool, error)
 	ListAllMiners() ([]address.Address, error)
 	ForEachClaim(func(miner address.Address, claim Claim) error) error
+	ClaimsChanged(State) (bool, error)
 }
 
 type Claim struct {

chain/actors/builtin/power/v0.go

@@ -115,3 +115,12 @@ func (s *state0) ForEachClaim(cb func(miner address.Address, claim Claim) error)
 		})
 	})
 }
+
+func (s *state0) ClaimsChanged(other State) (bool, error) {
+	other0, ok := other.(*state0)
+	if !ok {
+		// treat an upgrade as a change, always
+		return true, nil
+	}
+	return !s.State.Claims.Equals(other0.State.Claims), nil
+}

chain/actors/builtin/power/v2.go

@@ -115,3 +115,12 @@ func (s *state2) ForEachClaim(cb func(miner address.Address, claim Claim) error)
 		})
 	})
 }
+
+func (s *state2) ClaimsChanged(other State) (bool, error) {
+	other2, ok := other.(*state2)
+	if !ok {
+		// treat an upgrade as a change, always
+		return true, nil
+	}
+	return !s.State.Claims.Equals(other2.State.Claims), nil
+}