Reuse similar-looking CellStyles

[orestis]

A common issue when using Excel is that there is a limit of 64000 styles in an Excel document.

This means that at some point you need to reuse styles, you can't just be creating the same cell style and applying it. It gets worse if you're mutating already existing cell styles, e.g. to add a number format etc.

Apache POI has a CellUtil class that supposedly handles this, but during my testing it seemed buggy. What it does essentially is to just create a data structure out of a CellStyle, and iterate through the styles on the workbook to see if a class already exists.

Are you interested in adding a new function like (add-cell-style! cell style-map) that would do the same? Since create-cell-style! already accepts a data structure, the new function would support also generating the same data structure from an existing CellStyle.

[orestis]

(BTW I'm happy to provide a PR for this too!)