These refer to external functions and function signatures declared in
the preamble. Since we're already using the type names 'Signature' and
'Function', these entity references don't folow the usual EntityData /
Entity naming convention.
Entity references in instruction format operands also have member names
in the InstructionData struct. Track them the same way we track immediate operand member names.
Value operands still go in the arg / args members.
