If we did it all with `ArithmeticGate`s, the main loop (with ~101 iterations of cubing and a couple adds) would be fairly expensive, so this uses a (much smaller) custom gate called `GMiMCEvalGate` which does all the computations for one iteration of that loop.
I think this is the recommended way to apply Fiat-Shamir, to avoid any possible attacks like taking someone else's proof and using it to prove a slightly different statement.
Does this make sense? I think other libraries tend to include the leaf's index (either as an integer, or a series of bits indicating left/right turns) as part of a "proof". In FRI, the leaf indices are chosen by the verifier, so I thought that approach might be sort of redundant. Let me know what you think though.