Farming update

Updates seeds into comestibles, adds a minimum and maximum time for them to grow and randomizes growing, and cleans up some code using furniture.load_id in grow_plants in map.cpp.

Edit: I do still need to readd the old seeds for backwards compatibility and redo the item spawns, I realized.
> `calendar newcal = calendar::sync();`

Does not work, sync is a protected non-static function, if you want the current turn, use `calendar::turn`.

> `//maximum time it takes for a seed to grow`

What time? Seconds, turns, minutes, years? Please add the unit.

> `DAYS(seed.bday / 14400);`

This gives the birthday in turns, rounded down to midnight. Is this rounding needed, why not just `seed.bday`. In any case it's better to write `DAYS(1)` instead of 14400, so you don't need to comment the number (DAYS(1) == 14400).

> furn_name = furn.id;

Nitpicking: name and id is not the same. You use it correctly, but having a variable named '_name' but containing an id is a bit confusing.