Remove duplicate cuts at a node

[Ravenslofty]

While this doesn't improve quality of result in practice, it makes debugging easier and also reduces runtime a bit.

I also took the opportunity to replace assert calls with throws that won't get compiled out in release mode.

[Ravenslofty]

Okay, correction, this improves cavlc and max, but regresses arbiter and sqrt. However, multiplier, sin, priority and voter all map now.

[Ravenslofty]

Most of the above regressions solved by using std::unique from the standard library instead of rolling my own code; cavlc is regressed but the others are improved.