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.

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)
 }