No more Mr. BadHash.

Introduce a new intrusive, AVL-based file handle to cache
entry lookups.

The new dictionary remains a tree keyed on 64-bit hash of
file handle, with fallback to bitwise handle comparison, the
following details are changed:

* AVL linkage is intrusive, no consing of opaque buffers
* all hashing is by the 64-bit CityHash, no exceptions

Signed-off-by: Matt Benjamin <matt@linuxbox.com>