Adding the cache module.

Adding the cache module.
I'll be using the cache module to replace the name cache in mangle.c.  The
new one should be much faster and should require less memory.
Another feature is that the cache size can be limited by the amount of
memory used in addition to the number of entries allowed.  With the
current cache, the default is to allocate 12800 bytes representing 50
entries (256 bytes each).  With the same amount of memory, I should be able
to load over around two hundred entries.
Changes to the AVL trees were minor (missing comments).
Chris -)-----