New file for building-specific code.
Interface for Minister of Food.
Make enqueued buildings closed.
Gives buildings a number of workers, and a function to return its Building_datum.
Open areas when they finish construction.  City::get_total_housing() and City::get_military_supported() look at non-area buildings.  Lots of new data functions, many unwritten.
Better comments, lots of new data functions.
New function for Minister of Food.
Rename INFINITE_MINERAL to INFINITE_RESOURCE.  Add a Crop_amount struct, very similar to Mineral_amount.  Function for looking up and getting the name of Crop_types.