[struct_pack] better-read and develop,make code clean

### What happened + What you expected to happen
After reading struct_pack source code , I think threre some code need to be changed , to help readers and developers understande it better .

### Reproduction way
I will list what I think below

### Anything else
developers can read it and discuss together.
I have many question and suggestion for struct_pack, this issue will not close by me before i write down all of them

### Are you willing to submit a PR?

- [x] Yes I am willing to submit a PR!

Start by reading the struct_pack source code and review the author's listed questions and suggestions in this issue. The scope is not yet defined, so first identify specific readability or cleanup targets and discuss them with maintainers. Done should mean an agreed, bounded change with clear review criteria.

Written by the indexing model from the issue text.