It is possible to return multiple values from a function, so ReturnData contains
a VariableArgs instance.
We don't want return instructions to appear as 'return (v1)', so tweak the
printing of VariableArgs so the parantheses are added externally.
Add a typevar_operand argument to the InstructionFormat constructor which
determines the operand used for inferring the controlling type variable.
Identify polymorphic instructions when they are created, determine if the
controlling type variable can be inferred from the typevar_operand, and verify
the use of type variables in the other operands.
Generate type variable summary in the documentation, including how the
controlling type variable is inferred.
