Adds the `Form:set_as_inventory_for(player, [ctx])` API.
Also includes:
• `Form:unset_as_inventory_for(player)`
• Extends private `show_form` to account for inv edge cases
• Extends private `on_fs_input` to account for inv edge cases
• Adds several abstraction functions
• Add feature to `on_leaveplayer` to delete the inv formspec state as well
• Add a few unit tests for related features
