Call.FuncArg
Little abstraction over arguments: currently actual args, we'll want formal args later
type 'arg_payload t = {
exp : IR.Exp.t;
typ : IR.Typ.t;
arg_payload : 'arg_payload;
}
val map_payload : f:('arg_payload -> 'arg_payload') -> 'arg_payload t -> 'arg_payload' t
val arg_payload : 'payload t -> 'payload
val exp : _ t -> IR.Exp.t