[RFC] Perfect hash table plan

[Joerg Sonnenberger]

Hello all,
I've been looking at different indexing methods for use in different
parts of store and cache. One technique that I had part of the code base
around already is the generation of perfect hash tables. The plan can be
found in the Wiki:

    https://www.mercurial-scm.org/wiki/PhashPlan

The proof of concept doesn't include the generator as I've been too lazy
so far to port that code, but a small x86_64 Linux binary is included
with the Python glue around it for testing. The test case is the exact
matching for nodes to revision and I've outlined how the prefix matching
could be implemented as well.

Joerg