refactor: non-primitive handling of witness data

[Nashtare]

Following the discussion we had highlighting the current issues of having to provide all necessary inputs for non-primitive ops to the CircuitRunner before the actual run, here's my attempt at simplifying the barrier between Primitive vs NonPrimitive ops, as well as allowing a custom execution context for non-primitive ops to alleviate the issue above.

For now the way we pass private witness data is unchanged, but this should also ease a transition towards automatic handling at runtime, so that users don't need to pre-generate specific data for each instance of their non-primitive ops.

Main changes:

• merged all operations together, renaming Prim to Op to reflect it
• expression lowering now handles both kinds, although it outputs separate vectors for convenience (for future optimization passes and for faster iteration when filling up auxiliary traces)
• mapped to the (new) Expr::NonPrimitiveOp is an Op::NonPrimitiveOpWithExecutor, which contains
  • inputs (as Vec<ExprId>)
  • outputs (as Vec<ExprId>)
  • an associated ExprId (unlike the initial design not assigning any to non-primitive operations)
  • an executor, which defines custom logic specific to the operation to interact with the runner (so that it can get/set witnesses, execute its own internal logic and interact with private data (if needed))

There is still more work to allow for proper dynamic dispatch at runtime with minimum disruption but this isn't the main goal of this PR so I'd rather hold off any other changes.