gb_trees:enter/3
向树里插入（或更新）一个跟键相关的值
用法：
enter(Key, Val, Tree1) -> Tree2
如果键 Key 在树里不存在，则向树 Tree1 插入一个跟 Key 相关的值 Val，否则在树 Tree1 里更新跟 Key 相关的值为 Val。最后返回一个新树 Tree2。
Tree1 = gb_trees:empty(), gb_trees:enter(a, 1, Tree1).
Tree1 = gb_trees:empty(), Tree2 = gb_trees:enter(a, 1, Tree1), gb_trees:enter(b, 2, Tree2).
Tree1 = gb_trees:empty(), Tree2 = gb_trees:enter(a, 1, Tree1), gb_trees:enter(a, 2, Tree2).