circuit builder: more multiplication utils

[tcoratger]

Should be useful is general to simplify complex calculations and especially in #160 I think.

[LindaGuiga]

LGTM, thank you!

[tcoratger]

@sai-deng As discussed here #160 (comment) I've tried to improve a bit the quotient helpers.

• I've added a rich documentation to be sure everyone reading the code understands the logic here as the quotient polynomial in the circuit world is quite tricky.
• Let me know what you think about this.

[sai-deng]

This may not be related to this PR, but when I change a random line
let one = circuit.add_const(SC::Challenge::ONE);
to
let one = circuit.add_const(SC::Challenge::ZERO);
all tests still pass.
Shouldn’t test_fibonacci_verifier catch this?

[LindaGuiga]

This may not be related to this PR, but when I change a random line let one = circuit.add_const(SC::Challenge::ONE); to let one = circuit.add_const(SC::Challenge::ZERO); all tests still pass. Shouldn’t test_fibonacci_verifier catch this?

test_fibonacci_verifier doesn't get there because there is only 1 quotient chunk (log_quotient_degree = 1). Before the reorganization of the crates (commit b20ee94), there was a second test test_mul_verifier_circuit which catches the issue. So I think we should put that one back.