crypto/merkle: remove simple prefix (#4989)

## Description

This PR removes simple prefix from all types in the crypto/merkle directory.

The two proto types `Proof` & `ProofOp` have been moved to the `proto/crypto/merkle` directory.

proto messge `Proof` was renamed to `ProofOps` and `SimpleProof` message to `Proof`. 

Closes: #2755

consensus/msgs_test.go

@@ -33,7 +33,7 @@ func TestMsgToProto(t *testing.T) {
 	parts := types.Part{
 		Index: 1,
 		Bytes: []byte("test"),
-		Proof: merkle.SimpleProof{
+		Proof: merkle.Proof{
 			Total:    1,
 			Index:    1,
 			LeafHash: tmrand.Bytes(32),
@@ -211,7 +211,7 @@ func TestWALMsgProto(t *testing.T) {
 	parts := types.Part{
 		Index: 1,
 		Bytes: []byte("test"),
-		Proof: merkle.SimpleProof{
+		Proof: merkle.Proof{
 			Total:    1,
 			Index:    1,
 			LeafHash: tmrand.Bytes(32),

consensus/wal_test.go

@@ -128,7 +128,7 @@ func TestWALWrite(t *testing.T) {
 		Part: &tmtypes.Part{
 			Index: 1,
 			Bytes: make([]byte, 1),
-			Proof: merkle.SimpleProof{
+			Proof: merkle.Proof{
 				Total:    1,
 				Index:    1,
 				LeafHash: make([]byte, maxMsgSizeBytes-30),