Add runtime heuristics

[AlexanderViand]

@pjattke The ConeWriting paper [1] has a heuristic for runtime of FHE operations at certain depths, we should probably add this to get some fast rough estimates of optimisation impact without having to burn compute resources

[1] Aubry, P. et al. 2020. Faster Homomorphic Encryption is not Enough: Improved Heuristic for Multiplicative Depth Minimization of Boolean Circuits. Topics in Cryptology – CT-RSA 2020 (2020), 345–363.