ACIR: Call opcode

[vezenovm]

Problem

ACIR currently has no way to point to another circuit during execution. This restricts our ability to have function call stacks during execution and ultimately in proving.

Happy Case

Add a new Call opcode similar to below:

/// Calls to functions represented as a separate circuit. A call opcode allows us
/// to build a call stack when executing the outer-most circuit.
Call { 
        /// Id for the function being called to be used by the caller of execution
        id: u32,
        /// Inputs to the function
        inputs: Vec<Witness>,
        /// Outputs of the function call
        outputs: Vec<Witness>,
}

We should also consider just making this opcode a blackbox function as it will be up to the backend to decide how it handles the call opcode. It may be clearer though to keep the opcode as its own call.

Project Impact

None

Impact Context

No response

Workaround

None

Workaround Description

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Additional Context

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