crypto/merkle: pre-allocate data slice in innherHash (#6443) (#9447)

Cherry-picking PR #6443

#### PR checklist

- [x] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed

CHANGELOG_PENDING.md

@@ -22,6 +22,7 @@
 ### IMPROVEMENTS
 
 - [pubsub] \#7319 Performance improvements for the event query API (@creachadair)
+- [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)
 }