following situations are covered:
- empty block
- one Tx
- Merkle root of a block with odd Txs should not change with repeating
last one
- Merkle root is computed with combining Merkle root of left subtree and right subtree
- block witness is Merkle root of a block when setting first Tx
to zero.
Signed-off-by: soroosh-sdi <soroosh.sardari@gmail.com>