gh-67766: Improve struct.pack out of range error messages

[StanFromIreland]

• Issue: struct.pack error messages do not indicate which argument was invalid #67766

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[StanFromIreland]

Request: @meadori (Though I think the expert may be inactive :-( )

[skirpichev]

LGTM

I don't like you added argument index to the module state, but alternatives looks more complex.

CC @serhiy-storchaka

[encukou]

AFAICS, putting the argument index in module state is not thread-safe.

I think the best way to solve this would switching all packing operations to use a new per-operation state struct, which would include a (borrowed) pointer to the module state.

[serhiy-storchaka]

This is a wrong approach. A global variable for the argument index should not be used.