- pkg manifest is list of files

	/usr/bin/bash 1321321372198798

  plus provides, requires and version?

- keep history of installed packages/journal of package transaction,
  so we can roll back to yesterday, or see what got installed in the
  latest yum update.

- we build a cache of the currently installed set to service
  dependency inquiries fast:

	map from property to pkg (as hash) providing it
	map from property to pkgs requiring it
	map from pkg name to manifest
	map from string to string pool index

	no implicit provides? not even pkgname?

- properties are strings, stored in a string table

- on disk maps are binary files of (string table index, hash) pairs

- at run time, we mmap the map, and keep changes in memory in a splay
  tree or similar.  if searching the splay tree fails we punt to the
  mmap.  once the transaction is done, we merge the map and the splay
  tree and write it back out.

- the on-disk string pool is sorted and we keep a list of indices into
  the string pool in sorted order so we can bsearch the list with a
  string to get its string pool index.  maybe a hash table is better,
  less I/O as we will expect to find the string within the block we
  look up with the hash function.

- signed pkgs
- gzip pkg xml files somehow?