The `farm` global provides access to a
variety of information about your farm:

 `farm.height`: number of tile rows
 `farm.width`: number of tile columns
 `farm.tile(x,y)`: get tile info

Tile info includes `name`, `type`, and
other information about any object or
terrain feature at the given location.
For crops that includes `readyForHarvest`
and `minutesTillReady`.
