- Add code for 'remove' operation on iterators.
- Add rot_rl() and rot_lr() which perform the same jobs as calling
  rot_right/left or rot_left/right, but optimize the operation.
- Maybe change the AVL tree to be configurable in terms of the balance factor
  k in HB[k], although from my own testing performance _always_ degenerates
  when relaxing this value.
- Top-down splay tree?
- Optimize weight-balanced tree code.
- Implement skiplist.
- Write man pages.
- Write an install-time shell script that compiles all the libdict public
  header files into one header file so we don't have to have a whole directory
  in ${PREFIX}/include just for libdict.
