General Merkle Follow Up (#2510)

* tmlibs -> libs

* update changelog

* address some comments from review of #2298

crypto/merkle/proof_simple_value.go

@@ -25,7 +25,7 @@ type SimpleValueOp struct {
 	key []byte
 
 	// To encode in ProofOp.Data
-	Proof *SimpleProof `json:"simple-proof"`
+	Proof *SimpleProof `json:"simple_proof"`
 }
 
 var _ ProofOperator = SimpleValueOp{}