Create a minimal os.cairo program that executes a list of tx

[ClementWalter]

Why

The goal of Keth is to run a program to prove a block, ie to run a program that takes as input a state and a list of tx and execute them all sequentially.
On the other hand, the current kakarot codebase is designed to perform one single EVM tx triggered by a starknet contract call.

What

Implement a simple loop logic in the os.cairo file.