This patch refactors a whole lot in the file for complex solidify to make it more readable and maybe even extendible. There are no (intentional) functional changes apart from now using angle_normalized_v3v3 in the local utility at the top.
The new plan is to make the bug fixes on the version before this cleanup (current) to also be applied on 2.90 and to get this cleanup after all currently known bugs are fixed.
After the cleanup I will move the merging feature from solidify to the weld modifier as a simple/fast weld mode (rewriting it along the way).
This should make complex solidify faster and the weld modifier more versatile. It will also reduce the amount of complexity in complex solidify for maintenance and further extentions.