Filter

- Full transition to the new filtering mechanism, using the new expression
  evaluator. It is working well enough to start the dogfood.  Many tests
  now broken.
A3

- Removed obsolete ::extract_filter.
A3

- Removed obsolete ::find_limit method and _limit member.
A3

- Removed obsolete _read_only_command member.
A3

- Removed obsolete ::infix method.
A3

- Removed obsolete ::expand method.
A3

- Removed obsolete ::sequence method.
Parser

- Renamed A3t to Parser.