buildMerkleTree() is internal to the hash package and if anyone calls it with
`len(leaves) == 0` he deserves a panic. As it's the only error case in it, we
can remove error value return from this function and simplify NewMerkleTree().
Go's Hash is explicitly specified to never return an error on Write(), and our
own decoding functions only check for length which is gonna be right in every
case so it makes no sense returning errors from these functions.
