[new] Update main macros to use signal toolkit (but keep old handlers)

This commit:

  - Removes the old filtering implementation, and instead now relies
    on the filtering provided by `encore/signals`. The old filtering
    vars are now ignored, but temporarily retained.

  - Temporarily removes the docstrings for `p`, `profiled`, `profile`.
    This are being moved to new resource text files that'll be added
    in a forthcoming commit.

  - Temporarily retains old handlers and `HandlerVal` implementation.
    This'll be changed in a forthcoming commit.

Author: ptaoussanis