Extract flat lists of warnings from various design elements.
These functions append together the warnings from, e.g., all modules in a module list, to create unified lists of warnings.
Note: if you want to summarize or print warnings, a vl-reportcard-p is typically more useful than a flat list of warnings.
Note: these functions don't clean the warnings in any way, and so you may end up with many redundant warnings. Because of this, it is probably a good idea to clean-warnings before flattening.