input: Write a function to search for a character in a binary tree.
  assert tree.search('G') == True
  assert tree.search('A') == False
  assert tree.search('F') == True
output: class Node:
      def __init__(self, key):
          self.left = None
          self.right = None
          self.val = key
  
  class BinaryTree:
      def __init__(self):
          self.root = None
  
      def search(self, key):
          return self._search_recursive(self.root, key)
      
      def _search_recursive(self, node, key):
          if node is None:
              return False
          if node.val == key:
              return True
          return self._search_recursive(node.left, key) or self._search_recursive(node.right, key)