Rewrite evaluators.
Rewrite evaluators.

The evaluation is split in two steps:

1. Creating a sane AST from a filter criteria;
2. Evaluating the messages against the AST.

Step 1 is the most complicated, since GMail has many quirks that
they have to be mimicked with regular expresisons and expansions
(not done yet).
Step 2 is instead simple, since the evaluation itself will be
straightforward.

mbrt/gmailctl,mbrt/gmailctl