Integrate cffi-grovel

Added a groveller based on Dan Knap's and Matthew Backes's cffi-grovel
and Stelian Ionescu's fork iolib-grovel.  cffi-grovel is inspired by
SBCL's groveller.  This is a preliminary version.

Includes a new wrapper generator syntax to simplify the writing of
C glue libraries among other minor features.

- Updated TODO items related to grovelling.
- Integrated documentation into the CFFI manual.
- src/libraries: use ".so" as the default library suffix.