Update status effect management, etc.

Separated "status_effects" into "constant" and "temp" categories,
updated the "assign_minor_stats", "menu_draw_row_callback",
"menu_select_callback", and "equip_heavy_item" functions, added
"unequip_item_at" and "player_is_using_magic_type" functions, and made
various other minor changes to the code.