radix-tree: First drop of radix tree.

radix-tree: First drop of radix tree.

An implementation of an adaptive radix tree.  Has the following nice
properties:

  - At least as fast as the hash table
  - Uses less memory
  - You don't need to give an expected size when you create
  - It scales nicely (ie. no large reallocations like the hash table).
  - You can iterate the keys in lexicographical order.

Only insert and lookup are implemented so far.  Plus there's a lot
more performance to come.