An implementation of intrusive AVL trees in C.

Some notes:
 * These trees are "intrusive", which means that the memory for the
   tree nodes is managed by the caller and they are intended to be
   embedded inside the objects being tracked by the tree.
 * Despite this, the library *does* store opaque 'void *' pointers in
   the nodes so as to not require any macro chicanery to recover the
   tracked objects and to facilitate using them in a traditionally
   malloc()-y manner. I am not totally sure this is the right thing.
 * All of the library routines (except the consistency checkers) are
   non-recursive, making them suitable for environments with
   constrained stack sizes.
 * To facilitate non-recursive operation and amortized constant-time
   traversal, nodes store parent pointers.
 * In order to avoid duplicating lots of code for symmetric right and
   left cases, children are stored in a length-2 array that is indexed
   by `avl_dir_t`.
 * Trees support using different comparison routines for lookup/delete
   and for insert.
 * Nodes do store their full height and update it as necessary. This
   simplifies the code a *lot*, I think.
   To avoid pulling child nodes into cache spuriously, we cache child
   heights in parent nodes (which is like a 30% win in my tests).