Farming update 2

Started a new branch to clean things up. Gives different seeds different times to sprout, as well as adding a few new ones. Hopefully this should make farming a little more useful.
It looks like you reverted some of my map/item refactor stuff, these lines need to be something like:
```
 // Erase fertilizer tokens, but keep the seed item
i_rem( pnt.x, pnt.y, 1 );
auto *seed = get_item( pnt.x, pnt.y, 0 );
```
and thereafter seed is a pointer instead of a reference. (use -> instead of . to get at members).

Concerning naming, I'd keep the item called seed and rename the seed type to... seed_type:
```
it_comest* seed_type = dynamic_cast<it_comest *>(seed_item.type);
```
etc