Convert `DebugContext` to accumulator

[penelopeysm]

Don't think there's any real reason why it needs to be a parent context. The only meaningful things that it could wrap were LikelihoodContext and PriorContext and both of those will be gone, so I think Debug should itself be a leaf.

It would tie in with a general philosophy that:

• leaf contexts indicate different modes of model execution (i.e., being called by different functions)
  • it is nonsensical to stack multiple leaf contexts
• parent contexts indicate modifiers to the model itself (i.e., Condition/FixedContext change whether we go down assume or observe, PrefixContext changes the varnames themselves.)
  • multiple parent contexts simply mean multiple modifiers (which in general are not commutative, hence order matters)

(And parent contexts that were commutative have I think mostly been converted to accumulators.)

[penelopeysm]

Actually, scratch that, I think DebugContext should be an accumulator.

[penelopeysm]

Closed by #976