Implement SpookTrees

[VagrantBlackbird]

SpookTrees are trees that eat Gnomes. They are a means of blocking Gnomes.

• If a Gnome steps in front of a SpookTree, it gets eaten, unless the SpookTree is already eating a Gnome

• Eating a Gnome takes 3 turns

• The only way to pass a SpookTree is by feeding it a Gnome

• Actions on a SpookTree have no effect

• Giving water to a SpookTree has no effect

• Gnomes that are high are also eaten

[Siriah]

Name suggestion: "Chomping Willows"