Allow naval units to destroy land units

Since you can't fix the glitch of infantry walls over water, I propose that naval units should be able to break land unit walls so that you don't end up with invincible land walls in water in maps where there are no air units.

For example, you are playing GGG. You are invading natives as Spain because he took your American land. But wait! Natives built a land wall over water, I can't break them because this is the 18th century and there are no air units yet. Well, I guess natives are now invincible.