codegen_c_hdr.pl doesn't understand vectors of unions

[quietust]

This script has never properly supported vectors of unions, such as the "line_subject_target" field in the structure "poetic_form_part" - a recent change to DFHack/df-structures made said union externally-defined, causing the script to emit a TODO comment instead of a syntax error that I could track down and manually fix.

There are a few other things that need to be fixed as well - the size of stl-fstream on 64-bit Windows is now 280 bytes (int64_t[35]), and $vecpad should have been removed entirely since vectors aren't padded in 32-bit or 64-bit Windows (ever since Toady switched to Visual C++ 2015).