Create a balanced merkle tree for L2 to L1 Messages

[MirandaWood]

Currently, L2 to L1 messages are hashed into a merkle tree at the tx level, with the root of this tree being the out_hash calculated and output from the base rollup.

From the base upwards, each merge and root circuit accumulates this as out_hash = H(left.out_hash, right.out_hash).

Previously, each transaction was limited to just 2 messages, which meant the tx's out_hash = H(msg_0, msg_1). However, transactions with just 1 message (=> out_hash = H(msg_0, 0)) caused issues as methods in ts to recreate the out_hash were not padding with empty messages. This was fixed in #6959, but introduced a variable size tree for the tx's out_hash. With the possibility of wonky rollups, this tree would be very unbalanced and messy to keep track of.

A solution is to ensure all L2 to L1 messages in a block are formed into a balanced tree. We could:

• Keep track of the tree's state in the rollups like note and nullifier trees, using PartialStateReference to accumulate them
• Add L2 to L1 messages to the parity circuits

A rollup just provides the final out_hash (the root of the message tree) and the height (currently a height range since the tree has variable height) to the Outbox. Consuming this message requires a valid message preimage and path, which is checked against the stored out_hash using sha256.