Extend single-implementation to detect wedges

[PeterRugg]

Currently, we are careful to only generate valid instructions expected by the DUTs. This leaves gaps in not checking that illegal instructions are illegal, and don't wedge the core (as it turns out they did in Toooba's case). Some templates intended for single-implementation should allow us to test for this kind of bug without having to align the model and DUT on all possible instructions. Assertions could then be added that any unknown instructions trigger traps.