NmtHasher.EmptyRoot has unnecessary double append instead of creating slice with double capacity

[odeke-em]

If we look at

nmt/hasher.go

Lines 157 to 159 in 29e9433

	emptyNs := bytes.Repeat([]byte{0}, int(n.NamespaceLen))
	h := n.baseHasher.Sum(nil)
	digest := append(append(emptyNs, emptyNs...), h...)

we notice this code

nmt/hasher.go

Line 159 in 29e9433

digest := append(append(emptyNs, emptyNs...), h...)

but that code could be made clearer, simpler and even faster (if in hot loop) by

emptyNs2X := bytes.Repeat([]byte{0}, int(n.NamespaceLen)*2) 
h := n.baseHasher.Sum(nil) 
digest := append(emptyNs2X, h...)