Using Wheel data structures recursively

[jenspots]

The trie data structure contains binary trees, but requires them to be rewritten instead of importing from bst. This leaves a lot of room for improvement in terms of code duplication and performance. The main reason is the fact that we cannot yet let a bst implementation use bst* as a the generic type.

What we want is essentially to be able to do

typedef union trie_node {
  trie_node* next;
  LIBWHEEL_TYPE leaf;
  enum kind {
    INTERNAL,
    LEAF,
  };
};

#define LIBWHEEL_TYPE trie_node
#define LIBWHEEL_ALIAS trie_node

#include <wheel.h>

This way, we might be able to use the bst implementation in our trie.