feat(tx_tree): Build blocks transaction tree within the root rollup.

[Maddiaa0]

Currently, a transaction hash is created within the kernel circuits. This transaction hash is then emitted as the first Nullifier of each transaction. To build the transactions tree, we will need to take these nullifiers for the current block, then construct a tree from them.

The root of this tree will be included in the block hash, allowing users to prove against a transaction tree root in the past.

Tracked by: #849

[LHerskind]

Closing this for now since we can do it through the content_hash change that we have planned in the yellow-paper.