Tidying of list handling
Tidying of list handling

Most importantly:

  - Hooray inheritance!  Switch back to the inheritance model and stop
    bothering to short circuit for digest256

Also starring:

  - add_prefixes should be defined first
  - move a comment from lookup_prefixes to add_prefixes
  - document the format of the return value of lookup_prefixes
  - use temp variables to clean up readability in lookup_prefixes

mozilla-services/shavar,rtilder/shavar,mozilla-services/shavar