Support function pointers as callees

[jberthold]

Function pointers are used by some stdlib functions (example: max calls max_by with a function pointer of FnOnce type) and executed using the call_once function. Examples of such function pointers show that the called function is present in the items but not necessarily in the functions array in the Stable MIR JSON.

• The callee name needs to be present in the functions array (a change in stable-mir-json is required)
• The Call terminator needs to support determining the func from an argument (currently it only supports operandConstant)