Use key sorting instead of cmp()

- Get rid of pkgsort() usage for preferred variants.
- Concretization is now entirely based on key-based sorting.
- Remove PreferredPackages class and various spec cmp() methods.
- Replace with PackagePrefs class that implements a key function for
  sorting according to packages.yaml.
- Clear package pref caches on config test.
- Explicit compare methods instead of total_ordering in Version.
- Our total_ordering backport wasn't making Python 3 happy for some
  reason.
- Python 3's functools.total_ordering and spelling the operators out
  fixes the problem.
- Fix unicode issues with spec hashes, json, & YAML
- Try to use str everywhere and avoid unicode objects in python 2.