Implement iterator for binary_search_tree<T>

[domfarolino]

API-wise, both min() and max() should return iterators, and we should introduce the following methods:

• begin(): returns the result of min().
• end(): returns an iterator pointing to nullptr
• find(): returns an iterator pointing to the a found value. Similar to exists(), but with an iterator return value

A binary_search_tree<T>::iterator should support the following operations:

• Pre-increment
  • Should rely on a newly-introduced static binary_search_tree::inorder_successor
• Post-increment
  • ditto
• Pre-decrement
  • Should rely on a newly-introduced static binary_search_tree::inorder_predecessor
• Post-decrement
  • ditto

Given iterator increment and decrement support, does that imply we want to mark the iterator as a bidirectional iterator? See #146