def each(&block)
      node = @first_leaf
      while node
        break unless node.each(&block)
        node = node.next_sibling
      end
    end
def each(&block)
      node = @first_leaf
      while node
        break unless node.each(&block)
        node = node.next_sibling
      end
    end
Iterate over all entries in the tree. Entries are always sorted by the
 key.
 @yield [key, value]