Changed visibility of some structures to pub

[litcc]

Because of some special needs, I usually have to read a large number of standard table data for processing, I need to quickly read a large number of xlsx table headers for subsequent processing of some operations based on the matching headers, but loading the whole table takes too long;
Based on the source code, I found the worksheet_range_ref function, which reads Sheets through a reader;
So my idea is to adapt the worksheet_range_ref function to get only the header information of the first row in a single table;
During my implementation, the visibility of some of the internal structures of calamine bothered me, the

For example.

I want to use the above internal fields in an external crate

Or is there a better program?

[tafia]

I don't mind having some new functions to expose only what you need, feel free to open a PR.
If possible I'd like internal specifics not to leak to userspace.