AggLayer: maintain proper tracking of LER in the bridge via MMR frontier

[mmagician]

We have the Keccak-based MMR frontier functionality ready as a stand-alone functionality, but we still need to integrate this into the bridging-out flow:

• address the TODOs in bridge_out.masm
• use the double-word Array for storing the frontier in bridge_out component
• append the leaf and update the frontier

Note: this should only be done after the bridging-in flow is tackled.

[mmagician]

closed in #2424