There are no cases where Hash would fail, so don't return an error
value. Simplifies every caller, as can be seen from the test changes.

Fixes cosmos#757

The byte buffer in Node.hashWithCount is not necessary because the
hash.Hash interface implements io.Writer. Use that directly instead to
save allocations and memory.

There's no need because the hashing can never fail.

Tree construction never fails, so don't return an error.

Fixes cosmos#756