crypto/merkle: pre-allocate data slice in innherHash (#6443)

So we can reduce pressure on runtime for checking that slice has enough
capacity before appending.

CHANGELOG_PENDING.md

@@ -93,6 +93,7 @@ Friendly reminder: We have a [bug bounty program](https://hackerone.com/tendermi
 - [rpc] \#6265 set cache control in http-rpc response header (@JayT106)
 - [statesync] \#6378 Retry requests for snapshots and add a minimum discovery time (5s) for new snapshots.
 - [node/state] \#6370 graceful shutdown in the consensus reactor (@JayT106)
+- [crypto/merkle] \#6443 Improve HashAlternatives performance (@cuonglm)
 
 ### BUG FIXES
 

crypto/merkle/hash.go

@@ -22,5 +22,9 @@ func leafHash(leaf []byte) []byte {
 
 // returns tmhash(0x01 || left || right)
 func innerHash(left []byte, right []byte) []byte {
-	return tmhash.Sum(append(innerPrefix, append(left, right...)...))
+	data := make([]byte, len(innerPrefix)+len(left)+len(right))
+	n := copy(data, innerPrefix)
+	n += copy(data[n:], left)
+	copy(data[n:], right)
+	return tmhash.Sum(data)
 }