Modify LTChip to take Value<F> instead of F

[enricobottazzi]

The bug has been discussed here

The root cause of the problem is the design of the assign function from the LTChip gadget from the zkevm.

As you can see the witness assignment function takes as input lhs and rhs of typeF. Because of that this trick had to be performed in order to extract F from an assigned cell as pass it to the chip.

summa-solvency/src/chips/merkle_sum_tree.rs

Lines 338 to 350 in a7d0c1f

	total_assets_cell
	.value()
	.zip(computed_sum_cell.value())
	.map(|(total_assets, computed_sum)| {
	if let Err(e) = chip.assign(
	&mut region,
	0,
	computed_sum.to_owned(),
	total_assets.to_owned(),
	) {
	println!("Error: {:?}", e);
	};
	});

The problem of such implementation is that a circuit assignment function is conditional on a witness value being Some. This is dangerous because "the keygen synthesises the circuit without having access to the witness; whereas the prover runs synthesis with the witness. Since this closure is conditioned on witness values being Some, it will only execute in the prover, meaning we would end up with an inconsistent circuit structure cf. keygen."

• Create PR to Zkevm
• Modify our circuit accordingly